[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.
What do we want to do if there's a state to be trimmed out in the middle of a sequence? I think this could happen, for instance, if you're using a lag_time > 1.