Add documentation.
Use resize and [] instead of push_back on init.
Topic interface support, partial action support.

- Listening to command topics implemented.

- Listening to action goals is implemented, but goal completion (success or
  aborted) is not implemented yet. Requires joint tolerances to be in place.

- Add missing bits to make the plugin load.
QuinticSplineSegment is now inherently multi-dof.

- MultiDofSegment wrapper is no longer needed.

- Position, velocity, acceleration aata is now stored in a SoA structure, which is
  easier to manipulate.