Create spatial actuator issue62

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
Does it make sense for an `Actuator` to have this method? The interface for specifying the override force is in ScalarActuator. Also, this name is a little confusing to me; it's not obvious how `setOverrideForce` (override is a modifier/adjective) and `overrideForce` (override is a verb) are different. Though this might not be the time to address this.