BigBox dual IR Z max endstops

Dual Z endstops for leveling dual Z motors on E3D BigBox.

I recommend Duet Wifi for dual Z endstops with mesh bed leveling.

PiBot optical slot switch endstops:
http://www.pibot.com/pibot-optical-endstop-rev16.html

Duet config and macro can be found here: https://forum.e3d-online.com/index.php?threads/bigbox-dual-optical-z-max-endstops.2675/#post-29578