Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
`overrideForce` was removed from Actuator in a couple of commits ago and it's now only a feature of `ScalarActuator`, as it's a scalar method. So, it was safe to remove it from BodyActuator as well (just did it). 
Also, I made an issue to remember fixing the naming issue, from `force` to `actuation` in these methods. @aseth1