Additional pytest coverage

[pldorn]

Description

Additional python tests on the data generation/filtering pipeline. This PR includes new test coverage of:

• src/data/scripts/clean_and_pare_down_data_current_year.py
• src/data/scripts/clean_and_pare_down_data_all_years.py's new multiyear historic output

Fixes #100

Testing Instructions

• pytests are clean

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [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

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