Feature: Extend Segment Functionality

[matt-humphrey]

Summary

The purpose of this contribution is to begin extending the functionality of segments.
I've created a new segments.py which defines a function seg_group() which contains basic logic for allowing segments to contain groups of values.
I've made changes to validate.py to allow segments to handle None, and to handle seg_group().
Finally, I've created test_segments.py to test the different aspects of segments more thoroughly.

Related GitHub Issues and PRs

• Ref: fix: allow None/NA to be a valid segment #188
• Ref: feat: extend segmentation to allow grouping of values in a column #189

Checklist

• I understand and agree to the Code of Conduct.
• I have followed the Style Guide for Python Code as best as possible for the submitted code.
• I have added pytest unit tests for any new functionality.

[rich-iannone]

This is looking really good! Initial thoughts:

• add a docstring for seg_group() that also contains some examples (good model to follow is the starts_with() function in pointblank/column.py)
• add seg_group to a new __all__ list in pointblank/segments.py
• create a section and entry for seg_group() in docs/_quarto.yml; presumably there will be a few seg_*() helpers, so a Segmentation Helper Functions section seems like a good thing to do right now
• perform very basic tests of SegmentGroup and seg_group() (mostly just for coverage purposes, but it's useful as a sort of 'snapshot')
• later on, we may have to advertise the existence of the seg_*() helper functions in the docs of the validation methods (but that can wait until we have a good collection of them)

Once this is done, I think it'll be ready to merge! I think that additional seg_*() functions and addressing Windows test errors could be done in separate PRs.