Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
I wrote the clutchedPathSpring test case. It is testing that the force in the spring goes to zero when unclamped. Otherwise it would remain the weight of the block being supported. If it is no longer responding to its controls it would fail.  Maybe ClutchedPathSpring needs to be updated to match the changes in Actuator->ScalarActuator?