Adds a flag for returning the model spectra for SDSS BHM spec files

[havok2063]

This PR adds a model flag to the loader for SDSS BHM spec files to optionally return the model spectrum. The model is stored as a separate column in the same table with the flux spectrum. This flag sets the model data as the Spectrum "flux". There is an added metadata boolean keyword is_model whether the returned Spectrum object is a model or not. This is a placeholder implementation until specutils adds support for model spectra.

# returns original flux spectrum
flux = Spectrum.read(file, format='SDSS-V spec')
flux.meta["is_model"]
False

# returns model spectrum
model = Spectrum.read(file, format='SDSS-V spec',  model=True)
model.meta["is_model"]
True

Pinging @albireox for review as well.