Verify line flux algorithm for wavelength vs frequency space

[duytnguyendtn]

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.