Controlling Fetch robots with velocity

[kosta-jo]

For the Fetch environments, how to control robots with joint velocities rather than with offsets in Cartesian space from the current end effector position?

[jkterry1]

PR #2762 is about to be merged, introducing V4 MuJoCo environments using new bindings and a dramatically newer version of the engine. If this issue still persists with the V4 ones, please create a new issue for it.