RAD-136 Add Slope and Error to Dark

[PaulHuwe]

Resolves RAD-136

Closes #318

This PR addresses adding slope and slope error to the dark reference file for use in the ramp fitting step.

Checklist

• Schema changes discussed at RAD Review Board meeting
• Added entry in CHANGES.rst under the corresponding subsection
• Updated relevant roman_datamodels utilities and tests
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/408/

[codecov]

Codecov Report

All modified lines are covered by tests ✅

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[ddavis-stsci]

LGTM

[schlafly]

I think we also want to get rid of "err" as part of this PR. That's a 3D quantity that we're currently not using. I don't think we want both err and dark_slope_err.

[ddavis-stsci]

I thought the slope errors are just for the fitting. There are no other errors associated with this file?

[tddesjardins]

@schlafly is correct, we don't need both a dark_slope_err and err array. The error propagation in the end should look like:

total_var = var_rnoise + dark_slope_err + (dark_slope + f) x var_poisson

where f is the fitted slope in the science pixels. The dark reference file should have arrays for:

data (2-D, float32, units DN / sec)
dq (2-D, uint32)
dark_slope (2-D, float32, units DN / sec)
dark_slope_err (2-D, float32, units DN^2 / sec^2).