ULTRA l1b stat cull validation test

[lacoak21]

Change Summary

Overview

Add a validation test for the statistical outlier culling algorithm.

File changes

During the validation testing I realized I was modifying the std_diff array when recalculating the convergence after combining the flags. So that is the only part of the actual code that I had to update.

Testing

[Copilot]

Pull request overview

Adds a validation test for ULTRA L1B statistical outlier culling and adjusts the culling implementation to avoid mutating std_diff during the post-combine convergence recomputation.

Changes:

• Removed std_diff mutation during the combined-mask convergence recomputation in flag_statistical_outliers.
• Added an external-data validation test that compares culling flags/convergence/iterations/std-diff against precomputed results.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
imap_processing/ultra/l1b/ultra_l1b_culling.py	Stops overwriting std_diff during convergence recomputation after combining flags across energy bins.
imap_processing/tests/ultra/unit/test_ultra_l1b_culling.py	Adds an external-data validation test for statistical culling outputs.

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[subagonsouth]

LGTM