We need to be able to read the PFxB headers using the butler, and also to make the raw files useful for e.g. Fabrice.

The files are MEFs with pairs of HDUs (image, reference) so simply returning the first HDU as a raw read isn't helpful. Instead, return

(image[N-1] - reference[N-1]) - (image[0] - reference[0])

For real processing we'll use a more sophisticated reduction, which will require access to each HDU; please make these available via butler.get("rawb", ..., hdu=n)