Cleaning up WL Generation

[claalmve]

Description of PR

Summary of changes and related issue This PR cleans up the generation code in generate_gwl_tables.py by combining the 1850-1900 and 1981-2010 file generation time periods together, as well as handles the UserWarning of subsettings across different dataframes. It additionally is run correctly so that the generated ref and ref_timeidx files are correct.

Relevant motivation and context Code cleanup and generating the proper csv's for other tools to use.

Dependencies required for this change? None

Fixes # (issue), delete if not necessary None

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)
• [x] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. No tests, this is just a script.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules