JP-3102: NIRSpec combined MOS/FS processing

[melanieclarke]

Resolves JP-3102

Closes #7769

WIP: Process fixed slits defined in MSA metafiles, alongside standard MOS slits.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

Attention: Patch coverage is 51.98238% with 109 lines in your changes are missing coverage. Please review.

Project coverage is 58.90%. Comparing base (4179c09) to head (52359a0).
Report is 6 commits behind head on master.

Files	Patch %	Lines
jwst/pipeline/calwebb_spec2.py	5.33%	71 Missing ⚠️
jwst/master_background/nirspec_utils.py	50.00%	13 Missing ⚠️
jwst/nsclean/nsclean.py	0.00%	9 Missing ⚠️
jwst/master_background/expand_to_2d.py	63.63%	8 Missing ⚠️
jwst/wavecorr/wavecorr.py	79.31%	6 Missing ⚠️
jwst/combine_1d/combine1d.py	85.71%	1 Missing ⚠️
jwst/pipeline/calwebb_spec3.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8467      +/-   ##
==========================================
+ Coverage   57.97%   58.90%   +0.92%     
==========================================
  Files         387      388       +1     
  Lines       38830    39080     +250     
==========================================
+ Hits        22513    23021     +508     
+ Misses      16317    16059     -258     

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