[FEAT] support Spectrum1D objects with 2D flux arrays in Specviz

[rileythai]

Jdaviz component

Specviz

What is the problem this feature will solve?

Specviz currently does not support 2D flux arrays in Spectrum1D objects when loading data via the specutils default loaders.

As an example:

spec = Spectrum1D(spectral_axis=np.linspace(4000,6000,100)*u.Angstrom,flux=np.ones((4,100))*u.Unit("1e-17 erg / (Angstrom cm2 s)
"))
s = Specviz()
s.load_data(spec)

will load to the NotImplementedError.

NotImplementedError                       Traceback (most recent call last)
File ~/miniforge3/envs/sudev/lib/python3.10/site-packages/jdaviz/core/helpers.py:500, in ConfigHelper._get_data.<locals>._handle_display_units(data, use_display_units)
500         raise NotImplementedError(f"converting {data.__class__.__name__} to display units is not supported")  # noqa
NotImplementedError: converting NDData to display units is not supported

Describe the desired outcome

Since multi-dimensional flux can be passed into the Spectrum1D object (see here in specutils docs), it would be nice to have this automagically converted and imported into Specviz by jdaviz.

A potential implementation is to just convert all Spectrum1D objects with ND flux sent into the app into SpectrumList objects.

Additional context

This issue is related to astropy/specutils#1185 (comment) and astropy/specutils#1107 (comment) .