Rename isDrivingElement to isDrivingSceneElementPose

Rename isDrivingElement to isDrivingSceneElementPose in the
Physics::Representantation. This makes it more clear as to what is being
driven.
Require that SceneElements have a PoseComponent

Before we were just checking that there are no more than one
PoseComponent in a SceneElement. Now we check that there is one and only
one PoseComponent.