CrashProbe (Z auto leveling by Crash)

This is an experimental Z height probe using a hall effect sensor wired to a spare analog input port.
Sliding the retract bar drops the probe below the nozzle tip.
Before a print, a gcode will probe the bed and set the Z height.
After probing, when the nozzle approaches normal print height the magnet will automatically pull the probe up into it's storage position.
A Z end stop is no longer used and the probe will automatically adjust Z height to varying bed heights.

After configur