Standardize auxiliary input formats for pipeline steps

[stscijgbot-jp]

Issue JP-3850 was created on JIRA by Melanie Clarke:

It is sometimes necessary for pipeline steps to take in auxiliary input that does not neatly fit into simple parameter values.  For example, the resample step allows a custom WCS as input, passed as a file in ASDF format.  The ami_analyze step allows overrides for affine2d and bandpass parameters, passed as ASDF files. The cube_build step allows override offsets in RA and Dec per input file, specified in an ASDF file containing lists of filenames and offset values.  The tweakreg step allows custom catalogs per input file specified as a text file with columns (filename, catalog) or in the input association.  The skymatch step allows user-specified sky values specified as a text file with columns (filename, sky value) (as of JP-1649).

It would be helpful to develop a consistent policy for how we take auxiliary input from users for these kinds of tasks.  ASDF files are likely required for complex inputs like WCS objects. Should we require ASDF for all such inputs? If we allow text files for simpler input cases, should we require a standardized format?  When, if ever, should we require input from association files to be supported?

When we have answered these questions, it would be helpful to provide some standard utility functions to support reading in and validating auxiliary input to reduce code duplication.