What is claimed is:1. Safety switching device for electrically controlled machines used in combination with the control elements of the machine control-system itself during hand or manual operation, in which the multi-stage safety switching device has several channels or several switching circuits and the switching circuits have at least one operating element which can be adjusted by an operator depending on the desired switching function, characterised in that each switching circuit (7, 8) is assigned at least one position-sensing device (14, 15), comprising in particular contactless detection devices (16, 17) for detecting, at any time, the position (4 to 6) of the operating elements (2, 3) corresponding to the respective switching stages, which can be adjusted by the operator.2. Safety switching according to claim 1, characterised in that each switching circuit (7, 8) is assigned at least one electronic evaluating device (36, 37) for detecting or determining positions (4, 6) of at least one operating element (2, 3), which can be adjusted by the operator, the evaluation operations of the evaluating devices (36, 37) being independent of one another.3. Safety switching device according to claim 2, characterised in that the evaluating devices (36, 37) are built with electrical parts or components based on different technologies.4. Safety switching device according to claim 2, characterised in that the evaluating device (36) of the first switching circuit (7) is of a different circuit design than that of the evaluating device (37) of the second switching circuit (8).5. Safety switching device according to claim 2, characterised in that each evaluating device (36, 37) is connected respectively to at least one electric switch element (47 to 50) on the output side.6. Safety switching device according to claim 5, characterised in that at least one of the switch elements (47 to 50) has a protective circuit (69 to 72) to protect against overload and/or short circuit currents.7. Safety switching device according to claim 6, characterised in that the protective circuit (69 to 72) monitors the electric power conducted across the switch elements (47 to 50) and is connected to the evaluating devices (36, 37) and/or with the start and/or emergency stop outputs (43, 44; 45, 46) thereof.8. Safety switching device according to claim 5, characterised in that the switch elements (47, 48) of the first switching circuit (7) are based on a different technology from that of the switch elements (49, 50) of the second switching circuit (8).9. Safety switching device according to claim 5, characterised in that the switch elements (47 to 50) are contactless switching members, in particular semiconductor elements.10. Safety switching device according to claim 9, characterised in that the switch elements (47 to 50) formed by semiconductor elements co-operate with a device (59 to 62) for setting a standard flow direction of the switching current across the switch elements (47 to 50).11. Safety switching device according to claim 10, characterised in that the device (59 to 62) comprises a plurality of semiconductor diodes (63 to 66) in a bridge arrangement.12. Safety switching device according to claim 10, characterised in that the device (59 to 62) comprises two pairs of diodes (67, 68) connected in the same direction of current flow and the respective switch element (47 to 50) is connected to the two pairs of diodes (67, 68) in parallel.13. Safety switching device according to claim 12, characterised in that each pair of diodes (67, 68) comprises two semiconductor diodes (63, 66; 64, 65) connected in a same conducting direction and switch outputs (55 to 58) of the safety switching device (1) between the serially connected semiconductor diodes (63, 66; 64, 65) of each pair of diodes (67, 68).14. Safety switching device according to claim 9, characterised in that the switch elements (47 to 50) formed by semiconductor elements co-operate with a device (59 to 62) for setting a standard polarity of the switching current applied to the switch elements (47 to 50).15. Safety switching device according to claim 5, characterised in that the switch elements (47, 48) of the first switching circuit (7) are bipolar transistors (51, 52) and the switch elements (49, 50) of the second switching circuit (8) are field effect transistors (53, 54).16. Safety switching device according to claim 2, characterised in that each evaluating device (36, 37) has a start authorisation output (43, 44) and an emergency stop output (45, 46).17. Safety switching device according to claim 16, characterised in that the start output (43, 44) and the emergency stop output (45, 46) of each evaluating device (36, 37) is connected to a respective switch element (47 to 50).18. Safety switching device according to claim 17, characterised in that the emergency stop outputs (45, 46) co-operating with the switch elements (47, 50) of the first switching circuit (7) and the second switching circuit (8) are connected to operate in series.19. Safety switching device according to claim 18, characterised in that the parallel connected switch elements (47, 49) are connected in series to the serially connected switch elements (48, 50) and are connected to or isolated from a current supply across this switching array by means of at least one actuator, e.g. a relay.20. Safety switching device according to claim 17, characterised in that the start outputs (43, 44) co-operating with the switch elements (47, 49) of the first switching circuit (7) and the second switching circuit (8) are connected to operate in parallel.21. Safety switching device according to claim 2, characterised in that the electronic evaluating device (36) of the first switching circuit (7) is of a different technological structure from that of the evaluating device (37) of the second switching circuit (8).22. Safety switching device according to claim 2, characterised in that the first switching circuit (7) has an analogue evaluating device (36) and the second switching circuit (8) a digital evaluating device (37) for the electric signals, in particular light sensor signals, applied to signal outputs (38 to 41) of the position-sensing devices (14, 15).23. Safety switching device according to claim 2, characterised in that one of the electronic evaluating devices (36, 37) incorporates at least one threshold circuit and/or a Schmitt trigger circuit.24. Safety switching device according to claim 23, characterised in that the other electronic evaluating device (36, 37) has a plurality of digital gate arrays (42) for evaluating the signals of the position-sensing devices (14, 15).25. Safety switching device according to claim 2, characterised in that one of the electronic evaluating devices (36, 37) is a freely programmable gate array (42) with an evaluating logic programmed accordingly.26. Safety switching device according to claim 1, characterised in that each switching circuit (7, 8) is assigned at least two position-sensing devices (14, 15).27. Safety switching device according to claim 1, characterised in that each switching circuit (7, 8) is assigned its own independent operating element (2, 3).28. Safety switching device according to claim 1, characterised in that the operating elements (2, 3) are kinetically coupled with one another.29. Safety switching device according to claim 1, characterised in that the position-sensing devices (14, 15) are optical detection devices (16, 17).30. Safety switching device according to claim 1, characterised in that the position-sensing devices (14, 15) are light sensors (18 to 21).31. Safety switching device according to claim 1, characterised in that the position-sensing devices (14) assigned to the first operating element (2), provided in particular as first light sensors (18, 19), and/or the position-sensing devices (15) assigned to the other operating element, provided in particular as other light sensors (20, 21), are arranged in succession following a direction--arrow 34--in which the operating elements (2, 3) are adjusted.32. Safety switching device according to claim 1, characterised in that each operating element (2, 3), is kinetically coupled with a plate-type sliding member (30, 31) having an orifice (32, 33).33. Safety switching device according to claim 32, characterised in that the sliding members (30, 31) operating as optical apertures are mounted so that they can be adjusted relative to their respective light sensors (18, 19; 20, 21).34. Safety switching device according to claim 1, characterised in that the position-sensing devices (14, 15) are inductive and/or capacitative and/or pressure-sensitive or piezoelectric detection devices (16, 17).