JP-1693: Fix extract_1d for SourceContainer with 1 slit

[hbushouse]

Updated the branch of extract_1d that loops over slit models in an input SourceContainerModel to handle the case where there's only 1 SlitModel and hence it's not in the form of a list. This can occur in level 3 processing of WFSS observations, for example, when there's only 1 instance of a given source in all of the input exposures.

Fixes #5331 / JP-1693

[codecov]

Codecov Report

Merging #5439 into master will decrease coverage by 0.40%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #5439      +/-   ##
==========================================
- Coverage   71.74%   71.33%   -0.41%     
==========================================
  Files         414      414              
  Lines       37744    37985     +241     
  Branches     5842     5960     +118     
==========================================
+ Hits        27079    27098      +19     
- Misses       8946     9160     +214     
- Partials     1719     1727       +8     

Flag	Coverage Δ		*Carryforward flag
#nightly	71.73% <66.66%> (ø)		Carriedforward from ae5e686
#unit	52.46% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
jwst/extract_1d/extract.py	52.72% <50.00%> (-7.47%)	⬇️
jwst/regtest/conftest.py	70.77% <0.00%> (ø)

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[jdavies-st]

Would be nice at some point to refactor this (and elsewhere) to have more generic checks if they are a container type or not. This will require a small refactor of ModelContainer and our other container types, but that might be a better check rather than relying on the specific model classes for these checks. I.e. it will be more compatible with Roman which will have different DataModel subclasses.