Fixes some possible issues in delta printing code
Don't apply Z probe XY correction when probing at current point with G30
with a P parameter
Added experimental simulation code, controlled by M37 for now
Added X and Y z-probe offsets to G31 command
Reworked G31 and G32 to do less work in the step ISR and to support Z
probe on delta printers
Z probe modulation pin is now selected using M558 R parameter instead of
probe type