* Update to MuJoCo 2.1.0

- Remove license-related parts of the build process.
- Correctly parse the comments for array sizes based on nmocap.
- Explicitly exclude a few structs from generated wrappers, where
  the structs don't have a corresponding typedef.

Still TODO: Update README.md once a new package is pushed to PyPi.

* Remove MuJoCo key docs from README.md.

As of version 2.1.0, MuJoCo doesn't require an activation key.

- Adds PID-PI position/velocity loops.
- EMA filter is moved to the outer position loop in cascaded control.

* introduce an inverse dynamics (ID) controller. We select between PID vs ID custom controllers based on the actuator_user field that is defined per actuator. `user="1"` would trigger activating an ID controller where the gainprm are interpreted as Kp, Td, ema_smoothing_term, respectively.

* renaming

* Clean

* Cast to int

* wip cascade

* Enabled PID and cascaded PI control

* refactored PIDs

* fix id

* add some comments

* some more formatting

* test tweaks and fix to apply gravity compensation pre-clamping

* updated cascade documentation

* rm unused error term, move effort limit definition closer to where its used

* remove unused memory allocation

* simplify des_vel definition

Co-authored-by: rubendsa <ruben@openai.com>

Co-authored-by: Tao Xu <tao@openai.com>

* Add GCC9 and prefer newer over older versions

* Bump version

* a small fix

* make it or

Move from `lockfile` library to `fasteners`

* 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.