Make it possible to save a Transition() object

[thriveth]

These objects contain the most important information, and should as far as possible be possible to save the state of them.

Basic idea: Make everything a dict of str, int, float, bool, and dump to JSON, human-readable thingies first.

Astropy.quantities can be saved by doing a Q.value.dumps() (should be loaded by numpy into anarray when read). This would go in a dict together with the unit.to_string(), which can then be reconstructed with u.Unit(unitstring).

Should be possible and would be super useful, but would take a while to write.

[thriveth]

Given that the whole Summary thingie now round-trips (yay) to YAML, all the pieces should be there. Just a matter of writing the function that saves the state of an individual transition.

But do we even want that? I what use case would that be practical?