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)
We actually do not override `computeForce`. The way it works is that we override computeActuation if it is flagged to. Then we use the given value to `setActuation` and then use it to compute the `torque`/`force` which is going to be applied by the actuator. I guess renaming the variable `force` to `actuation` here makes it less confusing. Thanks for pointing it out.