The Gen3 formatter class for HSC raw data doesn't call setFilter on the returned Exposure, which is a problem for downstream processing.  To fix that, we need to:

• add a makeFilter to FitsRawExposureFormatterBase in daf_butler, similar to the existing makeWcs and makeVisitInfo (including using it in the base class readInfoComponent.
• implement makeFilter in obs_subaru appropriately.

We'll probably have to call the afw.image.Filter constructor with force=True to try to avoid mucking with the global variables (and hope no one else does); a better solution will have to wait for RFC-541.