U Can't Purify This

Adds additional boolean to mutations: "purifiable" : "false", means Purifier goes right over it, with naught but a faint itching sensation.  (The boolean defaults to "true".  No updates to mutations.json required.)

I took quite a lot of Rivet's time kicking around why it wasn't working, and am duly grateful therefor.  Thanks much, Rivet.

This'll start work on the Thresholds and eventually get Cata to that whole posthumanity thing.  Merge when ready, if you would, please.