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)
:) Should we apply it to all the codes at some point.. to keep the format consistent.