Make the NED module be able to query txt spectra

This is a feature request we received in an e-mail. It would be useful to have an example for such a file linked to this issue, too. Anyways I'm opening this issue now as is.

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The ability to pull fits spectra for a large number of NED objects is really awesome. I was trying to figure this out with wget commands for a long time then noticed you did this here.

However, many of the spectra on NED are in .txt files rather and not available as fits files. It would be great to have the ability to query for these txt files too so you don’t miss out on many of the available spectra.
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Start by locating the NED module and its existing FITS-spectrum query entry point. Review how a representative NED .txt spectrum would be identified and retrieved, then define completion around querying those files alongside FITS spectra and covering the behavior with tests; this issue does not provide an example file or test location.

Written by the indexing model from the issue text.