Purge typehints; mention type in docstrings

[jwallwork23]

Closes #121, Closes #127.

It is better practice to mention the types of arguments and return values in the docstring, rather than using type hints. Doing so ensures that this information ends up in the docs. (The metric module is not addressed because it is covered in #56.)

This PR also:

• makes a few methods and attributes private
• rewords docstrings for readability
• uses type(self) rather than self.__class__
• uses solver_kwargs rather than adj_kwargs
• gives TODOs issue numbers

Would you mind reviewing this @acse-ej321? Apologies that it is somewhat long and boring.

[jwallwork23]

Thanks very much for reviewing this, @acse-ej321! Good suggestions.

[jwallwork23]

Great, thanks again for reviewing this!