A bunch of OpenAI Gym environments

These envs return the feet positions and velocities in the info dict under feet_pos and feet_velp, as well as the same positions relative to the torso (rel_feet_pos and rel_feet_velp). All envs are parametric (can have their masses and lengths changed). In HalfCheetah, due to the way the model is parametrized, the parametric HalfCheetah is not identical to the original one (but is very similar). Instead, the GaitTrackHalfCheetahOriginal-v0 can be used if an identical model is needed (but it cannot have its link lengths changed).

Envs

1. GaitTrackAnt-v0
2. GaitTrackWalker2d-v0
3. GaitTrackHopper-v0
4. GaitTrackHalfCheetah-v0
5. GaitTrackHalfCheetahOriginal-v0