Prusa i3 Z-probe Sled Mount

This is a sled designed to carry the Z-axis endstop via an
arm with an adjuster for Z-height. The sled uses printable
LM8UU bearings to reduce weight.
The sled works via an electromagnet. When the X-axis moves
to MAX position, the electromagnet can be engaged to capture
the sled and thus tow it into position for Z-probing, or
disengage the electromagnet to dock the sled.
The electromagnet is engaged via a relay a pin on the Arduino (RAMPS).
The endstop mounts to the arm