Inverse Dynamics FeedForward

The Inverse Dynamics Feed Forward update to Baxter's Joint Trajectory Action Server (JTAS) is meant to 

- Create smoother interpolated paths between trajectory points that are passed into the server by using Bezier curves. These curves keep the interpolated trajectory continuous in 3 dimensions (position, velocity, acceleration) for each joint
- Allow faster movements when using JTAS by feeding forward the trajectory's velocities and accelerations commands into the motor controller
- Maintain compatibility with trajectories that supply position-only trajectory points
I've probably asked this before, but with a velocity mode, and a "desired position, velocity, acceleration" mode - is there a reason we don't update all the fields in desired (and potentially 'actual' when appropriate)?

In future, it could also be useful to get the SEAJointState (for the actuals) - maybe also useful to have that in the limb class...