add implicit integrator https://github.com/google-deepmind/mujoco/blob/e1e224892531c01ad2cb3ac3924d4e7362908e88/src/engine/engine_forward.c#L1094 this feature will require implementing mjd_rne_vel https://github.com/google-deepmind/mujoco/blob/e1e224892531c01ad2cb3ac3924d4e7362908e88/src/engine/engine_derivative.c#L596
add implicit integrator
https://github.com/google-deepmind/mujoco/blob/e1e224892531c01ad2cb3ac3924d4e7362908e88/src/engine/engine_forward.c#L1094
this feature will require implementing mjd_rne_vel
https://github.com/google-deepmind/mujoco/blob/e1e224892531c01ad2cb3ac3924d4e7362908e88/src/engine/engine_derivative.c#L596