Refactor remove legacy

[conorheins]

First major refactor of internal workings of pymdp, including:

• Removing Categorical and Dirichlet classes for representing discrete probability distributions
• Revised old TMaze notebooks.
• Added two new tutorial notebooks (refactored versions of those started by @BerenMillidge and @daphnedemekas)
• Removed the core library and pulled the main functional libraries (inference.py, learning.py and control.py) into the main pymdp module.