New DRO programs

I kept the X, Y and Z handles on my mill when I converted it to CNC. I moved the cables from my old DRO to a Mesa board as part of the conversion. I really missed the old DRO since I often do 1-off work that does not use the CNC capabilities of the mill.I wrote a couple of DRO programs, one that is tightly integrated into Linuxcnc and another that does the bare minimum necessary to mimc an old style DRO program. Both are written in Python and can be started via a hal file and run alongside axis.

See github.com/rgbond/yadro for the code on github.