Make matplotlib figures {{butler.put()}}able.

Jim Bosch says on Slack:
> Basically, that involves grepping daf_persistence for FitsCatalogStorage, copying what you see and calling it MatplotlibStorage, and then adjusting it appropriately (i.e. call savefig instead of writeFits, raise an exception when trying to read).

Paul Price notes that there are some gotchas involving the fact that the backend has to be set immediately after import, and this could prove tricky, especially if people want to combine this with pop-up style debug plots.. Some relevant info on this might be found in DM-14159.