[WIP] Implement PCCA and PCCA+

At this point, I think the key idea is to see whether we like the style of how this works / looks / flows.  I'll add tests once we agree on the API etc.
Okay, so PCCA doesn't actually have a macrostate transmat. I guess. Even though I argued for this option, I agree with @schwancr that it's a little confusing. Can we buff up the docstrings to help smooth the transition?