Make var_flat optional.

[schlafly]

This PR makes the ELP not compute var_flat by default for individual exposures. It adds a new keyword "include_var_flat" to the flat field step spec to enable including the flat field variances, with a default value of False---we will not by default compute var_flat. It adds tests to make sure that this keyword is being followed.

Together with spacetelescope/rad#462, it makes the mosaic pipeline either create a var_flat extension or not, depending on whether the input exposures contain var_flat. If not all of the input exposures include var_flat, it will not compute var_flat in the output.

This will lead to changes in the regression tests, since we will not be computing var_flat by default. The derived errors will also be different. If we update the exposures used as input for the mosaic pipeline to not include flat variances, the derived mosaics will also be different.

"err" will no longer include a contribution from var_flat by default, in either the exposures or eventually the mosaics. In principle we could include var_flat in "err" but still delete var_flat by default. This seems a little perverse to me, but I mention it as a possibility.

It makes a handful of forward-looking changes to the regression tests, essentially not requiring that var_flat exist.

@tddesjardins , @hcferguson, you've expressed opinions about removing this in the past; care to weigh in?

Tasks

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  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.00%. Comparing base (8601da8) to head (9eaecf7).
Report is 4 commits behind head on main.

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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1438       +/-   ##
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- Coverage   78.38%   62.00%   -16.38%     
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  Files         118       76       -42     
  Lines        7680     3190     -4490     
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- Hits         6020     1978     -4042     
+ Misses       1660     1212      -448     

Flag	Coverage Δ		*Carryforward flag
nightly	62.00% <ø> (-0.12%)	⬇️	Carriedforward from f9ebf61

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