Inverse mapping of Joint angle_vel

Hi all,

I was wondering if there was a method for doing the inverse of the Joint angle_vel method, i.e., go from DOF position and velocities back to a QP. These should be a 1-1 mapping as long as you have access to the root states. It seems like its performed when getting the default QP of a system here:
https://github.com/google/brax/blob/a09f3e75390c1ff017565ba2acfa77e838535fd3/brax/physics/system.py#L119-L180

It might be nice to have that as a dedicated method, since this gives a more succinct representation of the state. Let me know if I missed something somewhere.

Thanks!