Add functionality to save/load

[drbenvincent]

The pymc-experimental repo contains a ModelBuilder class which can help in productionized / deployment situations because of the save/load functionality.

It would be nice to bring this functionality in to CausalPy.

We should also add a general how-to docs page to illustrate its use.

I don't think we should inherit from the ModelBuilder in pymc-experimental. The functionality is relatively simple, so we could just add it to the PyMCModel class.

Very open to hearing from people using CausalPy in production on whether this would help in a production setting.