Read in IRAF FITS format (including multispec)

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I have [some code](https://github.com/andycasey/smhr/blob/master/smh/specutils/spectrum.py#L838-L995) (that @alexji and I maintain) that reads in IRAF FITS format of various types (chebyshev, legendre, polynomial, etc) that I'd be happy to update and create a pull request to bring it into the astropy ecosystem. It handles various dispersion mappings and multispec formats.

Would this be useful? If so, am I right in assuming that I should introduce it as an adapter in the specutils/wcs?

Start by reading the existing implementation in smh/specutils/spectrum.py#L838-L995 and inspect the specutils/wcs area mentioned in the issue. Clarify with maintainers whether IRAF FITS support, including dispersion mappings and multispec formats, belongs as an adapter; done means an agreed integration path and support for the listed IRAF formats.

Written by the indexing model from the issue text.