UPDATES:
 - extracted 0.0001 as a symbolic constant
 - replaced a few groups of (X, Y, Z) discrete doubles with
TGCodeOffsetSpec structs
 - updated GCODE_MACHINE_POSITION() to also take structs
 - implemented moving_axis_math() to detect immobile axes so that they
are not radius compensated -- covers the case when asking for one axis
to move would cause more than one (that) axis to move
 - temporarily added more debugging to radius compensation, will get it
running smoothly any day now ;-)