Mushroom-rl

- Fixed loading of alpha parameter in the SAC algorithm

- Implemented record interface to record videos of environments
- Updated MuJoCo interface with support for multiple environments XMLs
- Updated MuJoCo viewer with headless rendering, different backend support, advanced functionalities and options, and multiple view support.
- Improved SAC algorithm.
- Bugfixes and code cleanup

Minor release with bugfixes and improvements:
- fixed MuJoCo viewer window scaling on MacOS
- improved polynomial features and Gaussian radial basis functions
- new ProMP policy added
- Fixed bug in BoltzmannTorchPolicy, now the policy can be used properly with PPO and TRPO
- minor bugfixes in serialization

Minor changes to the MuJoCo interface:
- Updated to support the latest version of MuJoco [2.3.2](https://github.com/deepmind/mujoco/releases/tag/2.3.2)
- Added support to reset MuJOCo environment states using an observation

* Removed every Cython dependency, the package is easier to install now!
* Removed the humanoid environment, which depended on Cython
* Improved PyBullet environments
* New MuJoCo interface using native Deepmind MuJoCo bindings
* New air hockey environments implemented with MuJoCo
* The core now collects environment info and this info is passed to the fit method of the agent. This breaks the previous MushroomRL interface but allows supporting different kinds of algorithms (e.g. safe RL approaches)
* Improvements in the documentation
* Minor updates and bug fixes

- Added plotting functionality, previously from MushroomRL Benchmark
- Fixed MuJoCo interface
- Added missing discount factor to eNAC update
- Gym Real-time rendering
- Pybullet interface now enforces joint torque limits