Add UVW Axes for 9 Axis Control

[aldenhart]

Add UVW axes as secondary linear axes, with UVW corresponding to secondary XYZ axes, respectively.

• Extends motor mapping:
  • 1=x, 2=y, 3=z [primary linear axes]
  • 4=a, 5=b, 6=c [rotary axes]
  • 7=u, 8=v, 9=w [secondary linear axes]
• Adds three new axis objects: U, V and W (obviously)

[aldenhart]

Closing this issue with PR #305.

The addition of UVW adds three more linear axes, allowing up to 6 linear axes in a configuration. This simplifies using an axis as an extruder filament feed motor or similar function. (Recommend using W, as it's a secondary Z).

This release treats the secondary axes as independent axes; there is no linkage between corresponding axes - i.e. movement in U does not affect position of X, nor Z affect W.

Please treat this release as experimental and report issues or feature enhancements.