Added constraint relaxations for Bullet for hinge and prismatic
1. Fixed a bug where when the physics engine is changed on load it reruns the catalog plugin code. The problem is that some of the plugins could have physics engine specific objects within them, like relaxations. So when you add a new part it clones the cataloged plugin object and uses that, but if the physics engine specific object within it does not correspond to the currently selected physics engine then that is a problem. By re-cataloging them it ensures that they are correct.
2. Found that I was not calling initialization code for several of the joint types.
3. I also added a generic pointer for the 6dof spring to the joint type for bullet so I could start moving some generic code related to it down into a common base class.
4. Changed the damping of the bullet relaxation so that it was more consistent with vorttex. Large values provide more damping. Internally, I take 1/damping and use that for the actual damping value. Setting damping to 0 actually sets damping on joint to 1 for no damping.
5. Got the constraint relaxation axises for RPRO, hinge, and prismatic correct.