Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
That's right. I agree it would be better / more generic to have the forces in ground frame. Body forces were initially expressed in the body frame as that was needed for the specific problem we started with (applying contact forces to feet). I changed this to `ground` frame and mentioned it in the comments. For the specific uses, we can do the transform before passing the forces to the bodyActuator.