This PR changes "Force" to "Actuation" for all relevant Actuator's member functions.

(to make naming more consistent and descriptive; including functions such as getForce, setForce,
computeOverrideForce, overrideForce, isForceOverriden, setOverrideForce, getOverrideForce)
Sounds good Chris! and for not using `signal` with `actuation` here, sure! That makes sense and makes it less confusing (also mentioned by Ajay). So, I'll remove it from here and all other places it's used. @chrisdembia