Quaternion difference in rotation reward

[P-Schumacher]

It's not mathematically sound to directly substract quaternions as they follow their own algebra. This was used to compute a rotational reward here: https://github.com/MyoHub/myosuite/blob/4d146e1eec27b617111cb8c032107986849bf893/myosuite/envs/myo/myobase/walk_v0.py#L363

It should be replaced by a proper quaternion difference.