fix poisson calculation

[mwregan2]

Resolves JP-3463

This PR addresses a bug in the original PR #243. The addition of the dark current noise in the original PR was not the correct version. This PR fixes that and we now get the correct Poisson variance.

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:

Project coverage is 84.95%. Comparing base (af5aefb) to head (4d429d0). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #254 +/- ## ========================================== - Coverage 85.18% 84.95% -0.23% ========================================== Files 35 35 Lines 6797 6846 +49 ========================================== + Hits 5790 5816 +26 - Misses 1007 1030 +23 ```

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