Add gyroscopic term to physics integrator

Looking at the code in `integrator.py`, I did not find the right equation of evolution of the angular velocity (`ang` in the code)

According to Featherstone (and a derivation of mine and my university's textbook), the correct equations of motions should be :

With tau being the external torques, omega the angular velocity of the body, and I the inertia matrix in the local frame.

I guess is does not make much of a difference when most degrees of freedom are constrained, but clean math should be better.

Any plans to include this version of the equations in Brax ? And if no, can you give me some pointers as to how one might go and implement that ?