cleanup: JointState::_transform is now in model-frame

The JointState::_transform member used to be in the "avatar frame" (rotation to world-frame + translation by avatar position).  We now compute and store it in the model-frame only.  It is a simple matter to transform this data into the world-frame.

Comments or special variable names have been added to those parts of the code where we do joint-math in non-model-frames (such as: world-frame, parent-frame, or bind-frame).

The motivation for all of this cleanup is to make it easier to do a rag-doll simulation in the model-frame.