Fix unit conversion of limits from Fnu to Flam

[astrofrog]

Description

I have expanded the test suite for to_unit and found that an error occurred when converting limits from Fnu to Flam. The issue in this specific case was that if '_pixel_scale_factor' in spec.meta: was True so the clause relating to len(values)==2 was never invoked, resulting in an issue with incompatible dimensions.

In addition to fixing this logic, I have made it so that we just use one wavelength/frequency in this case for the conversion, as it doesn't specifically make sense to use the first and last spectral axis value. When converting limits we are probably interested in the values being right at the start of the spectral axis.

This builds on top of #2888 and should be rebased once #2888 is merged.

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)

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• 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)?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (b3c3ae9) to head (c26c5e1).
Report is 174 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2889   +/-   ##
=======================================
  Coverage   88.70%   88.70%           
=======================================
  Files         111      111           
  Lines       17132    17134    +2     
=======================================
+ Hits        15197    15199    +2     
  Misses       1935     1935           

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[gibsongreen]

Post-rebase of pixel scale factor updated PR: #2860:

• New limits test passing
• small medication to conditional to include update from this PR:

  jdaviz/jdaviz/app.py

  Line 148 in c26c5e1

  elif len(values) == 2:

• Profile-viewer behavior post display units/conversion changes functioning

[gibsongreen]

Note:

• 3.12 Test failures (11 total) same as Fix unit conversion from flux to surface brightness #2888