tjenness: you have a good point about testing the Formatters separately. I'm already doing that, and I think I was concerned about the various other magic that happens in the gen2 Mapper. But we don't have to worry about that in the gen3 world, since there aren't overrides for everything and all the "magic" happens in the Formatter itself. The conversion uses `ingest`.
Further update: I've got conversion of some DECam data mostly working, except for a slew of warnings on read and not getting the correct types for things (e.g. Defects being Catalog not DefectList). So, I think we're pretty close!