Refactor the code to be a pip package

[saynomoo]

Currently the only way, afaik, to play around with custom environments is to implement them to games directory of this repo. A more modular way would be to restructure the code so that it is a pip installable package, which can be then accessed in other environments (independent version control, easier python environment control, etc).

Afaik, this would be quite straight forward, couple of steps that I've recognized:

1. Organize the code like in pip package: https://python-packaging.readthedocs.io/en/latest/minimal.html
2. Make MuZero class independent of the internal package structure, for example by providing the Game and MuZeroConfig to the constructor.

Would this make sense?