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)
Could we change this to `This is the force or torque provided by this actuator divided by its optimal force.` Actuation `signal` can get confusing with Controller's, which send control signals. So I think we might should avoid using `actuation` with `control`.