Define how mkrefs.py and individual reference file creation scripts will interact

[bhilbert4]

Specifically, what will be passed back and forth between them? Once we define this, development can start on some of the reference file scripts, even if mkrefs.py isn't ready for them yet.

Should mkrefs.py just provide a list of input filenames, and leave it up to the reference file script to figure out which are darks which are flats, and pair them up if needed? mkrefs.py already takes in certain information though, like the maximum time allowed between files to be paired up. So that implies that that information needs to be passed along as well, or mkrefs needs to pair the files and pass those pairs to the reference file scripts. The complication with this method though is that each reference file type needs it's own inputs (e.g. a list of darks, or pairs of darks, or pairs of darks and pairs of flats, etc).

Also, if mkrefs is the script making the decisions about input patterns, that makes it harder to integrate a new module for a given reference file type, if that module happens to use a different set of inputs.

@arminrest, what do you think?