jointcal.py originally had this line:

        # Determine default filter associated to the catalog

        filt, mfilt = list(andConfig.magColumnMap.items())[0]

The preceding comment is incorrect, and jointcal should be able to deal with different filters in the refcat.

I've changed it to use the most common filter from the input catalog (as parsed by loadSkyCircle), which is slightly less bad, but we need to figure out what to do when someone passes dataRefs with multiple filters to jointcal. Jointcal should be refactored to use the filters of the exposure being fit when accessing the reference catalog (i.e., to bundle the exposures and refcats appropriately by filter).

I've added a note referencing this ticket to the relevant part of jointcal.py