Edited LinuxCNC driver to have some comments that explain how to bring it up.
Added primitive KiCAD support.
Added G21 to force LinuxCNC into millimeter mode.
Merge remote-tracking branch 'upstream/develop' into develop

Up to date with develop brach on home repository.
Separate out G21 command.
Merge branch 'develop' of https://github.com/waynegramlich/openpnp into waynegramlich-develop