Allow ingesting PFSD (guider) files

We need to ingest PFSD files, for the sake of butler access. These are different from PFSA files in two obvious ways:

• There are six guide cameras, and each PFSD file will contain N*6 HDUs, for N exposures taken with all six cameras. We will want butler access to the Nth set of images, etc.

• The filenames PFSDvvvvvvss.fits do have a 6-digit visit (matching the running SPS/PFS visit), but instead of an SM and an arm for the last two digits there is a two-digit "batch number" within the visit. The batch number is, unfortunately, indirect: it will increment from 00..99, but within each PFSD file there can be 1..N actual exposures. We currently envision N <= 5 (umm, 5 * 100 5s exposures will let us directly link the guide images for a ~40min visit).