Add a PypeIt loader to SpecUtils

[tbowers7]

As discussed in #1263 and related conversation among the development team, it would be helpful to add a loader for PypeIt spec1d FITS files to the Astropy-coordinated SpecUtils package.

I know various PypeIt developers and users have created their own SpecUtils "custom loader" for PypeIt data, but having a PypeIt-sponsored default loader would bring closer integration and expose the broader community of SpecUtils users to PypeIt.

I have built a working loader for PypeIt MultiSlit 1D spectra in a SpecUtils fork. The custom loaders others have built may be more elegant, but this is functioning for my needs (works with my own LDT/DeVeny data and Cooked Keck/DEIMOS data).

How to move forward:

• Discuss whether the PypeIt team wants to support a loader for 1D spectra into SpecUtils in the first place.
• If yes, should PypeIt create its own fork of SpecUtils to develop the full-featured loader and submit the SpecUtils PR -- or should such a loader come from one individual associated with PypeIt?
• If this moves forward, we will need a test in PypeIt that ensures the output from the pipeline is always loadable in SpecUtils via the default loader.
• From those who are more familiar with the Echelle and IFU pipelines, loaders for these 1D data will be needed.

I am willing to be a coordinator for this issue, but will need assistance from those who are more intimately familiar with PypeIt and its data products.

[tepickering]

having some familiarity with specutils, i'd be happy to help with this. i'll try to take a look at your specutils fork later today...

[profxj]

I support this and can help where needed. Meet on Slack to discuss further

[kbwestfall]

Where are we on this? Should we revive the discussion?

[tbowers7]

Added in #1562 .