Propagate variance arrays into SlitModel used as input for ResampleSpecStep

[jdavies-st]

Addresses #5798
Addresses JP-1944

Description

This PR allows propagation of the variance arrays of the inputs to ResampleSpecStep. Previously these were being dropped on the floor when converting an ImageModel into a SlitModel, which happens in the case of MIRI LRS.

Regression tests pass on this branch.

This is of course needed if we are to use the variance arrays for weighting and to compute uncertainties in the resampled products.

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[codecov]

Codecov Report

Merging #5941 (72eab2e) into master (c5cfcd5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5941   +/-   ##
=======================================
  Coverage   77.91%   77.91%           
=======================================
  Files         403      403           
  Lines       35359    35392   +33     
=======================================
+ Hits        27549    27575   +26     
- Misses       7810     7817    +7     

Flag	Coverage Δ		*Carryforward flag
nightly	77.91% <ø> (ø)		Carriedforward from c5cfcd5
unit	56.03% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
jwst/resample/resample_spec_step.py	90.51% <100.00%> (-4.67%)	⬇️

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[hbushouse]

LGTM