Persist nirDark cubes, and wire in a proxy to IsrTask

We need to save and read dark cubes for the H4. These correspond to the CCD pfsDark masterDarks, but with one image HDU per NIR read.

I propose:

• name them nirDark-$cam-$visit0 or similar
• name the individual image HDUs IMAGE_$N, with N starting at 1.
• allow for additional HDUs before the IMAGE_N ones.

The PfsIsrTask needs a proxy to one of these, and it will seek into the nirDark to get the read matching the length of the exposure being corrected. [ The first such seek takes about three seconds for a 900s ramp. ]