Add joint freezing through direct QP equality constraint

[mattrobotcontributor]

For one configuration, locking specific joints is required. Mujoco states that removing certain joints from the model is preferred over locking out joints through equality constraints. It is non-trivial to do that for a specific setup, but this change adds simple joint velocity equality constraints that provide exactly what I am looking for and figured others may find this useful.

[kevinzakka]

Hi @mattrobotcontributor, apologies for the delayed review. I’ll try to get to it by next week.

[mattrobotcontributor]

Thanks @kevinzakka. I do see this issue and think this is a cleaner solution than damping or deleting the joints and more correct than zeroing out the selected output velocities.