Fix auxiliary speeds bugs in physics.mechanics.

We fixed:
1) Auxiliary forces were not being formed correctly when there were nonholonomic (velocity) constraints. This is now fixed and a more thorough test of auxiliary forces and dependent speeds in one problem has been added. This test also checks the other outputs of KanesMethod against calculations done by hand.
2) In KanesMethod, the _mat_inv_mul method had a bug in temporary symbol generation - symbols were being generated that were not unique. This issue has been fixed.