DM-419

Forgot I could now create GitHub PRs for reviews.  Here it is; hope it's helpful.
We could, I suppose.  That seems harder, because it won't always be a one-to-one mapping between new fields and old fields.  It would also make it less useful for the immediate (but more temporary) purpose of easing the transition to meas_base.

If we were to translate the old catalogs into new ones, I think it'd work best as a script you'd run to convert an entire data repository, at which point we could force the user to provide additional information (e.g. via config) to clear up any ambiguities in the mapping.  But I'd rather not worry about that until the transition is over.  It may have been a mistake to work this hard on this kind of backwards compatibility from the beginning, but I definitely don't want to switch to a different strategy right now.