Improve readability of `generate_gwl_tables.py`

[claalmve]

Description of PR

Summary of changes and related issue

Reviewed generate_gwl_tables.py and made code more readable in certain locations.

Relevant motivation and context

We needed to document the GWL year generation process and make it more interpretable by developers.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Definition of Done Checklist

Practical

• [x] 80% unit test coverage
• [x] Documentation
  • [x] All functions/adjusted functions documented in the readthedocs.
  • [x] Documentation is pushed
• [x] Complex code commented
• [x] Naming conventions followed
  • [x] Helper functions hidden with _ before the name
• [x] Context of function is clearly provided
  • [x] Intent of function is provided
  • [x] How to test, so that it is not siloed on scientists and anyone can review
  • [x] Appropriate manual testing was completed
• [x] Any notebooks known to utilize the affected functions are still working
• [x] Linting completed and resolved

Conceptual

• [x] Doesn't replicate existing functionality
• [x] Aligns with general coding standard of existing functions
• [x] Matches desired functinonality from users/scientists