Refactored RigidBody.py so that mass is now a function of state, rather than a state variable itself.  This particular change is so that we can have different constituent mass components (separate fuel tanks, for example).
Fixing fallout from changing mass to a function argument; all tests currently pass.  Also added pseudo-torque for torque-free precession of asymmetric objects.  I need a good test case for torque-free precession