Dark correction needs to check ref file params more thoroughly

[hbushouse]

I've found a case where the dark current step inadvertently applies data from a dark reference file that's totally inappropriate. Right now the step first checks to see if NFRAMES and GROUPGAP in the science and reference files are an exact match. If they're an exact match, it applies the dark ref data directly. If they aren't an exact match, it grabs and averages frames out of the ref file. The way the code is constructed, it contains an implicit assumption that the science data will always have NFRAMES and GROUPGAP values that are greater than the reference data. I've discovered a case where the selected ref file has NFRAMES=4, while the science data have NFRAMES=1, and the code blindly goes ahead and tries to apply it. This is incorrect.

A check needs to be added to the dark correction step to make sure that the values of NFRAMES and GROUPGAP for the ref data are always at least equal to or greater than the values for the science data. If not, a warning should be issued and the step skipped.