Forcing One PyBullet Client per Environment

[goobta]

I guess that we didn't know how to PyBullet worked when we started. When trying to train the models in a env and then creating a new gui-based environment for rendering, we found that we had run into a lot of issues.

The only way to circumvent them was to restart the kernel, which also happens to be the only way to terminate the pybullet physics server. I'm gonna conjecture that when we create a second environment, it causes conflicts because there already is a simulation running on the PyBullet server. I believe if we forced each environment to maintain its own physics client, then we should be a-ok.

It's either that or find a way to train without having to render and be able to start rendering mid-pybullet session.