Our pickled skymap definition files are now largely a Gen2 relic, and it'd be nice to get rid of them in favor of putting the skymap configuration in a registry blob field for a number of reasons:

• it removes a pickled dataset
• it improves how defaulting of skymap dimensions at butler construction works in practice
• it opens the door a bit to registry not actually needing to have tables with rows for each patch

I'm envisioning a daf_butler ABC for both BaseSkyMap and Instrument, and a way to associate a governor dimension with a class or factory method, with the registry dimension record holding at least a Python qualified name (as instrument already does) and optionally some state (as skymap needs).

This will involve schema changes.