Gain calculator plug-in

[bhilbert4]

This module should be able to plug in to mkrefs and will be used to create gain reference files.

[bhilbert4]

I have a functioning module at the moment that creates a gain map from a pair of flats and optionally a pair of darks. This is using @arminrest 's dPTC method. Will need to integrate with mkrefs once that module has the proper hooks defined. Also need to create a script that will create a mean gain image given a list of gain images. We also have a script for taking a gain image and saving in CRDS format.