Verify line flux algorithm for wavelength vs frequency space

This concern was brought up by @PatrickOgle, which spawned https://github.com/spacetelescope/jdaviz/pull/1564 in `jdaviz`. There was a question brought up by @kecnry whether this should be moved upstream into specutils, so I'm filing this ticket to encapsulate that discussion.

In short, depending on how the flux units are defined, the spectral axis is converted to the appropriate space (wavelength/frequency) before performing the line flux calculation. @PatrickOgle could probably give more detail if requested.

Start by reviewing jdaviz pull request #1564 and the discussion about moving the work into specutils. Trace how line flux calculation handles flux units and converts the spectral axis between wavelength and frequency space. Done means the expected behavior is agreed and verified for the relevant unit definitions.

Written by the indexing model from the issue text.