Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
> uncommenting testClutchedPathSpring caused testActuators to fail on that case. Was it purposely commented out because it was failing for you? Was it purposely commented out because it was failing for you?

Ah, good that you brought this point up @aseth1. Yep, in fact building `testClutchedPathSpring` was fine but there was an "exception" while running it, occurring at line 506 (which I dunno why I can not comment on it directly):
``
// is unclamped and block should be in free-fall
ASSERT_EQUAL(model_force, analytical_force, 10*integ_accuracy);
``
I was not sure about this test-case design and commented it to check with its author later. Do you know who was working on this test case?