JP-3643: Add background error propagation for non-IFU spectral extraction

[ianyuwong]

Resolves JP-3643

Closes #8522

This PR adds background error propagation when subtracting the background during non-IFU spectral extraction by adding the Poisson/read noise/flat variance contributions from the background to the corresponding arrays from the source extraction region.

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• 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
• All comments are resolved
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.02%. Comparing base (4179c09) to head (84e65e6).
Report is 321 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8532      +/-   ##
==========================================
+ Coverage   57.97%   58.02%   +0.05%     
==========================================
  Files         387      388       +1     
  Lines       38830    38982     +152     
==========================================
+ Hits        22513    22621     +108     
- Misses      16317    16361      +44     

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[hbushouse]

Started a regtest run at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1501

[hbushouse]

Regression tests showed expected changes in error columns in x1d/x1dints products, and a bunch of unrelated changes due to a MIRI ref file update. So this looks good.