Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
No it doesn't make sense for the base Actuator to have this. The override force is defined to be a scalar so it should only be a "feature" of the ScalarActuator. It would be good to replace the use of force with actuation here as well. We can leave these changes for another PR, however.