Model & subtract scattered light between spectra

Be able to model the scattered light in the frame and then subtract off of the model. This could also be appropriate to move to ccdproc

The issue does not name any files, tests, or entry points. Start by defining how scattered light should be modeled and subtracted, then determine whether the work belongs in specreduce or ccdproc; done means the model can be applied to the frame and its scattered-light contribution subtracted.

Written by the indexing model from the issue text.