DM-12979 introduced open-shutter darks to correct the "Eye of Y" feature in HSC y-band images. For significant disk space gains, the open-shutter darks have been compressed in a custom FITS format. The code introduced in DM-12979 reads these by grabbing the filename with dataRef.get("yBackground_filename") and reading them using a function. Instead, these data products should have their own class with a readFits method so that we can simply dataRef.get("yBackground"). This will allow them to be treated more like actual calibs in the future.