update moment map plugin to use correct display flux units for moment 0

[cshanahan1]

This PR updates the moment map plugin to reflect if the current display unit is a Flux or Surface Brightness unit for moment 0 calculations.

[javerbukh]

If I create a moment map in the cubeviz example notebook, then convert to Jy/sr and overwrite the first moment map, I still see MJy/sr in the coords display. The moment map plugin itself looks correct and has the correct units, so if updating coords info is out of scope this is looking good! The one maybe bug I found is that when you first open the moment map plugin, there are no output units selected. If you press calculate, however, it still gives you a result.

[kecnry]

I think the unit stuff needed for this won't be released until 4.0, so this probably won't need to be backported or milestoned to 3.10.2 - is that right?

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.70%. Comparing base (8f65073) to head (4160c62).
Report is 176 commits behind head on main.

Files	Patch %	Lines
...configs/cubeviz/plugins/moment_maps/moment_maps.py	88.46%	3 Missing ⚠️
jdaviz/core/validunits.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2877   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files         111      111           
  Lines       17086    17123   +37     
=======================================
+ Hits        15156    15189   +33     
- Misses       1930     1934    +4     

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[cshanahan1]

@javerbukh I both of those concerns - the non-selection still allowing you to calculate a MM is now fixed, and the units of the displayed and / or saved MM should reflect the correct display units now.

[javerbukh]

Code looks good and the points I made earlier have been addressed. Nice work!

[gibsongreen]

Everything tests out and looks good to me!

[cshanahan1]

thanks for the reviews, i just rebased and ill merge this once tests pass