JP-3244: Provide auto-centering option for slit-like data containing point sources

[tapastro]

Resolves JP-3244

This PR addresses the inaccuracy inherent in using the user-provided TARG_RA/TARG_DEC as source location descriptors for defining the extraction region. The code submitted here collapses the 2D bounding box along the dispersion axis, fits a function to the resulting spatial extent (here a gaussian, but this could be changed), and uses the fit to determine the cross-dispersion extraction bounds.

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

Patch coverage: 4.76% and project coverage change: -1.47% ⚠️

Comparison is base (d1a60ee) 76.60% compared to head (8c7f6c1) 75.13%.
Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7763      +/-   ##
==========================================
- Coverage   76.60%   75.13%   -1.47%     
==========================================
  Files         456      456              
  Lines       36950    38277    +1327     
==========================================
+ Hits        28304    28758     +454     
- Misses       8646     9519     +873     

Flag	Coverage Δ		*Carryforward flag
nightly	77.43% <ø> (-0.01%)	⬇️	Carriedforward from 779e3e6

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

Files Changed	Coverage Δ
jwst/extract_1d/extract_1d_step.py	77.27% <ø> (ø)
jwst/extract_1d/extract.py	76.47% <4.76%> (-2.17%)	⬇️

... and 13 files with indirect coverage changes

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