JP-3077: Change the Computation in Ramps with only One Good Group in the 0th Group

[kmacdonald-stsci]

Resolves JP-3077

This PR addresses the incorrect usage of group_time for ramps that have non-zero groupgap and the 0th group the only good group in the ramp. Because of the readout pattern, the time for the 0th group alone is computed using (NFrames+1)*TFrame/2, instead. The code changes for this are in STCAL (spacetelescope/stcal#142). Those changes affected JWST testing of ramp fit, which have been addressed in this PR.

This PR should not be merged, until the STCAL PR gets merged.

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

[hbushouse]

Looks good.

Can you also add an entry to the jwst change log that references the change to ramp_fitting in stcal (i.e. references the stcal PR). Need to have a record of it over here in jwst, since it does affect jwst ramp_fitting.

[hbushouse]

We'll obviously want to wait to merge this until the stcal PR has been merged and a new stcal release is available.

[hbushouse]

Regression test running at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/570

[dmggh]

Looks fine to me.

[hbushouse]

Regression test run showed only unrelated failures due to a recent update of MIRI ref files.

[zacharyburnett]

We'll obviously want to wait to merge this until the stcal PR has been merged and a new stcal release is available.

stcal 1.3.4 includes the analogous changes to stcal: https://github.com/spacetelescope/stcal/releases/tag/1.3.4

[codecov]

Codecov Report

Base: 77.90% // Head: 77.91% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (efe4b56) compared to base (beb200d).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7461   +/-   ##
=======================================
  Coverage   77.90%   77.91%           
=======================================
  Files         455      455           
  Lines       36975    36912   -63     
=======================================
- Hits        28806    28760   -46     
+ Misses       8169     8152   -17     

Flag	Coverage Δ		*Carryforward flag
nightly	77.89% <ø> (ø)		Carriedforward from beb200d
unit	50.39% <ø> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
jwst/associations/lib/utilities.py	85.71% <0.00%> (-2.39%)	⬇️
jwst/ipc/x_irs2.py	10.46% <0.00%> (-1.03%)	⬇️
jwst/saturation/x_irs2.py	44.18% <0.00%> (-0.65%)	⬇️
jwst/associations/registry.py	99.40% <0.00%> (-0.60%)	⬇️
jwst/tweakreg/astrometric_utils.py	88.52% <0.00%> (-0.37%)	⬇️
jwst/associations/association.py	91.35% <0.00%> (-0.32%)	⬇️
jwst/associations/lib/process_list.py	84.29% <0.00%> (-0.26%)	⬇️
jwst/lib/signal_slot.py	81.39% <0.00%> (-0.22%)	⬇️
jwst/transforms/models.py	94.11% <0.00%> (-0.17%)	⬇️
jwst/associations/lib/product_utils.py	94.73% <0.00%> (-0.10%)	⬇️
... and 15 more

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