Fix metric arc tolerence to same value as imperial

Also add an ini file field and allow user variance of arc tolerances
within preset limits.

New PR following from #402 , original closed

Limits are currently set MAX_TOLERANCE_INCH / MM at 10x defaults.
This allows all the testing CAM code I have to run happily

If usage suggests other limits should be used, it is a very simple matter to change the define
in interp_internal.hh