Optical runout sensor for BMG Extruder

1.75 filament runout sensor utilising optical endstop, tested and working (on my machine, cheap Chinese endstop)
Mounts in the "hotend slot" in BMG Dualdrive extruder

Requires two M3x18mm screws

### There are absolutely 0 tolerances designed in, if your printer is not calibrated you might have trouble inserting endstop in to the body


Includes two versions:
* Short - most compact one, may cause some trouble with retracting filament when triggered
* Long - Bigger one, but circumvents problems mentioned above