Enhance find_outliers and identify_outliers performance by avoiding duplication and filtering columns

[d33bs]

Description

This PR enhances find_outliers and identify_outliers performance by filtering to only the necessary columns and avoiding data duplication where possible. Along the path towards this PR I also analyzed code coverage and found we needed to clean up a bunch of legacy fixtures which are no longer used here (should boost the coverage percentage overall now as a result).

Closes #134
Closes #86

What kind of change(s) are included?

• Documentation (changes docs or other related content)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

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• I have read and followed the CONTRIBUTING.md guidelines.
• I have searched for existing content to ensure this is not a duplicate.
• I have performed a self-review of these additions (including spelling, grammar, and related).
• These changes pass all pre-commit checks.
• I have added comments to my code to help provide understanding
• I have added a test which covers the code changes found within this PR
• I have deleted all non-relevant text in this pull request template.