Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
This question is unresolved. `overrideForce` is now split between Actuator and ScalarActuator. `Actuator` has a pure virtual overrideForce, but its implementors don't have the infrastructure to do anything with it. Is that intended?  If so, there should be some documentation for this.