Set up __getstate__ and __setstate__ and add serialization tests

[lmcinnes]

Most things should just pickle, however numba based attributes (functions and types such as numba.typed.List) need special handling with __getstate__ and __setstate__ methods to handle conversion of numba bits and pieces.

To aid in this we should have tests that all classes can be pickled and unpickled successfully. This will help highlight any shortcomings.

See also #62

[cakiki]

What exactly would this entail? I could have a look

[lmcinnes]

MOstly it just means converting any numba things to python things in __set_state__ and vice versa in __get_state__. We would be delighted if you wanted to make a PR.