Have QuantityModel support spectral-equivalent inputs

Right now the `QuantityModel` (as output by the fitting machinery) only accepts inputs with the *same* units as the spectrum.spectral_axis that was used to create it. It would be nice if it also recognized spectral equivalencies - i.e., if you give a continuum model, say, a `GHz` quantity, it should know to convert it if the input to the model is `angstrom`, using the `spectral` equivalencies provided by astropy.

Start with QuantityModel as produced by the fitting machinery and trace how it validates inputs against spectrum.spectral_axis units. Check the existing spectral-equivalencies behavior for a GHz input against an angstrom input, and consider the work done when those equivalent units are accepted.

Written by the indexing model from the issue text.