JP-3463: Add average dark current to dark models and ramp model

[tapastro]

Addresses JP-3463

This PR adds necessary datamodel updates to hold the average dark current, a parameter being added for tracking contributions to the poisson variance from the dark current.

Checklist

• [x] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [x] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e8a8f9e) 64.84% compared to head (4d7c3a6) 64.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #265 +/- ## ======================================= Coverage 64.84% 64.84% ======================================= Files 103 103 Lines 5694 5694 ======================================= Hits 3692 3692 Misses 2002 2002 ```

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