JP-2067: Add units to combine1d product table

[tapastro]

Closes #6002

Resolves JP-2067

Description

This PR adds units to the CombinedSpecModel table output of the Combine1D step. Flux and SB units are checked from the input spectra, while wavelength units are set to um.

Checklist

• Tests
• Documentation
• Change log
• Milestone
• Label(s)

[hbushouse]

Errors/differences in CI / SDP dependencies tests are due to use of updated NIRCam specwcs ref files (in latest context). Some assign_wcs tests need to have their truth values updated. Unrelated to this PR.

[codecov]

Codecov Report

Merging #6082 (b3bf8a2) into master (b77df8a) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6082      +/-   ##
==========================================
+ Coverage   77.13%   77.16%   +0.03%     
==========================================
  Files         402      402              
  Lines       34321    34444     +123     
==========================================
+ Hits        26474    26580     +106     
- Misses       7847     7864      +17     

Flag	Coverage Δ		*Carryforward flag
nightly	77.13% <100.00%> (ø)		Carriedforward from b77df8a
unit	55.19% <100.00%> (?)

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
jwst/combine_1d/combine1d.py	89.46% <100.00%> (-0.40%)	⬇️
jwst/associations/asn_edit.py	66.66% <0.00%> (-0.75%)	⬇️
jwst/associations/lib/process_list.py	73.49% <0.00%> (-0.59%)	⬇️
jwst/fits_generator/template.py	16.98% <0.00%> (-0.27%)	⬇️
jwst/timeconversion/time_conversion.py	10.96% <0.00%> (-0.08%)	⬇️
jwst/datamodels/make_header.py	91.38% <0.00%> (+0.02%)	⬆️
jwst/associations/lib/product_utils.py	94.82% <0.00%> (+0.09%)	⬆️
jwst/regtest/conftest.py	90.96% <0.00%> (+1.20%)	⬆️
jwst/tso_photometry/tso_photometry_step.py	77.41% <0.00%> (+1.61%)	⬆️

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