Moment map units match surface brightness+spectral units

[bmorris3]

Note: This PR is in draft mode to begin discussion until the next release of specutils. Before this PR gets merged, we need a specutils release, and to bump the specutils minpin in this PR.

Description

Recent Unit Conversion plugin updates led the units of the zeroth moment map to convert to the surface brightness unit on the y-axis of the spectrum-viewer. Previously, POs had decided on the default units of the zeroth moment map, which do not always match these "conventional"/"preferred" units.

This PR restores that preferred behavior for the moment map, and simplifies the logic used to get there.

Change log entry

• [ ] Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts, list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• [ ] Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• [ ] Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• [ ] Do the proposed changes follow the STScI Style Guides?
• [ ] Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Did the CI pass? If not, are the failures related?
• [ ] Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• [ ] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[kecnry]

is specutils 1.16 (released last week) what is needed here?

[pllim]

what is needed here?

A serious rebase. Lots of conflicts.

[bmorris3]

This PR is now failing on tests related to fixes that @cshanahan1 is working in her latest ticket on surface brightness in units of pix^-2. Waiting for progress on JDAT-4520.