Ragdoll Part 6: muscle constraints for driving Ragdoll and also damped velocity integration for verlet simulation

No longer using linear puppet strings for driving avatar animation.  Instead using constraints that are updated each frame.  This makes all the movement happen in the PhysicsSimulation which makes for more consistent joint placement after collisions are resolved.

Also added some (heavily damped) velocity interpolation for the verlet integration.  Without the damping the avatar wiggles a lot.

No improvement for tunneling through ragdoll limbs, but will work on that next.
I uncommented the simple-slaving and got rid of the more complicated parent-relative-slaving below.  We don't use this slaving anymore but I want to keep it in case we ever need it to unravel a skeleton that has become tied up in constraints.