[LinuxCNC/linuxcnc Issue#611] Support Arc Blending with UVW axes

Merge existing UVW-blending work into master:
– Generalizes canon / motion geometry calculations for 6D arc blend (XYZUVW)
– Replace usage of PMCartesian in many places with Vector6 type (to store XYZYUVW positions)
– Refactor canon / motion internals in several places to make XYZUVW velocity the “fundamental” velocity used by TP (instead of just XYZ)

Unfortunately, this work does not include blend capability for ABC axes, but it removes some of the obstacles towards that.