Propulsion System Actuator Model

[kylekrol]

Currently, the orbit models only take in an impulse vector directly in the coordinate system they're operating in. This is fine for tests with a tightly closed loop; however, for tests integrated with flight software in HOOTL or HITL we want PSim to be able to handle thruster on times from flight software as an impulse.

The easiest way to support this behavior is probably to add an actuator model "layer" than runs before the truth model. We could prefix it's state fields with actuators.prop.* or something similar and have a model take in thruster on times and output an impulse vector to the orbit model.