Using stall detection when loading filament

[dc42]

I finally found time to test a feature that I first implemented in firmware 1.20, which is to use stall detection on extruder motors to feed filament into the hot end until resistance is encountered. I am pleased to say that it seems to work, with one minor bug that can be worked around.

I have written instructions cfor using it at https://duet3d.dozuki.com/Wiki/Stall_detection_and_sensorless_homing#Section_Using_extruder_stall_detection_during_filament_loading.