Rename "Force" to "Actuation".

(to make naming more consistent and descriptive; including functions such as getForce, setForce,
computeOverrideForce, overrideForce, isForceOverriden, setOverrideForce, getOverrideForce)
Doxygen style comments in the .cpp file don't make any sense. Internal comments describing implementation details would be OK. These are legacy, so feel free to delete!