Fix CytoDataFrame custom attributes in label_outliers

[d33bs]

Description

This PR fixes CytoDataFrame custom attributes in label_outliers by adding the custom attributes after processing.

Closes #165

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

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• 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.

[jenna-tomkinson]

LGTM! 🎉

[d33bs]

Thanks @jenna-tomkinson, merging this in!