* PID implementation inside mujoco-py

* addressing the comments from Jerry and Arthur

* add unit tests for new PID control

* add one more unit test for backward compatibility

* addressing comments

Also point to mujoco-bin for licenses

* Reraising mujoco error exceptions

Mujoco 2.0.2.3 

* Initialize perturbations properly.

* Expose "set constants" mujoco operation.

* Updated README documentation.

* Removed the 'alpha' pre-release version info.

* Fixing pip install command that seems to help with docker build in CI.

* Updating README to the next major version.

* improvements to sim.ray()

* typo

* group filter is bytes

* Add build directory to the git ignore file.

* Updating mujoco-py for mujoco 2.0

* Removing commented out test.

* Remove windows support.

* Alpha version prerelease.