Devel (Arc Support + Documentation + Machine Definition Tweaks)

- Support for arcs (Delta robots only. Cartesian bots will still compile, but error at runtime when issued a G2/G3 command). Arcs must currently be < 180*.

- Much improved documentation in the kinematics sections.

- Machine details are exposed via functions, instead of public typedefs. This allowed for the removal of lots of otherwise unnecessary templatization.

- Build system automatically detects any platform-specific *.cpp files under `drivers/<platform>`.

- Increased DMA bus priority for more stable event timing.