JP-3482: Load units from FITS default header keywords directly

[tapastro]

Resolves JP-3482

Closes [spacetelescope/jwst#8109]

This PR addresses a lack of specified units in the spec/spectable jwst schemas, resulting in metadata not propagating through a save/load cycle. Additionally, loading a spectrum into a datamodel will not have any unit strings accessible. This feels like hacky workaround, but does generate unit strings in the spec_table. Looking forward to expert review 🙂

Checklist

• [x] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [x] updated relevant milestone(s)
• [x] added relevant label(s)

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (12a4c3a) 64.71% compared to head (e5dfeb8) 64.62%.

Files	Patch %	Lines
src/stdatamodels/jwst/datamodels/multispec.py	35.29%	11 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #238 +/- ## ========================================== - Coverage 64.71% 64.62% -0.09% ========================================== Files 102 102 Lines 5668 5685 +17 ========================================== + Hits 3668 3674 +6 - Misses 2000 2011 +11 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[hbushouse]

Is this being superseded by #243 and hence can be closed?

[braingram]

I believe the open PR #242 supersedes this (I'm not sure if the test PR #243 fixes the issue here and is more an example of one way to define units to dicuss in issue #240). @tapastro does #242 look good to you?

[braingram]

Not enough coffee yet this morning... I clicked merge on #242 and then realized I hadn't yet heard back on my last comment. @tapastro does #242 solve the issue and if so are you ok with closing this PR?

[tapastro]

This has definitely been superseded and can be closed!