Make Model a Python class by moving Cython parts into a new _Graph class

[arcondello]

See also #174

Some highlights:

• Most of the pure-Python methods have been moved to Model rather that _Graph
• _Graph doesn't hold onto States, Model does
• Several methods (e.g. .lock, .unlock, .minimize) have "basic" features implemented in Cython but some bits are implemented in Python.
• Keep the cimport-ability of Model via some aliasing tricks (need to contemplate how much I like this one a little more)

[arcondello]

A few things left to do:

• Better document the alias thing if we want to keep it
• Add typing to model.py
• Another pass over the docstrings for clarity

[arcondello]

Upon reflection, I think it makes sense to keep Symbol.model as model rather than say renaming it Symbol._graph. Even if we were to add an Expression class calling expressions "model" is IMO not such a bad thing. If it later becomes confusing we can always do the rename in a different PR.

[wbernoudy]

This and maybe to_networkx are the only methods I believe that we'll want to have on the future Expression class that we would duplicate. Considering how short this is and that to_networkx is not necessary for now (if ever) seems fine to me.