Have QuantityModel support spectral-equivalent inputs

[eteq]

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.

[nmearl]

We also need to evaluate the need for QuantityModel as astropy 4.0 introduced a refactor of the model fitting infrastructure.