Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
Would be good to describe the convention of the applied spatial force as {force, torque} and that the controls must have the same ordering. Also that the force and torque are expressed in the Body frame. That wasn't obvious until i read the implementation. I would have guess they would be expressed in ground. Any reason to choose the body frame?