Fix: Make `CostDistribution` `df` parameter private

[BenTaylor-TfN]

Describe Changes

Fixes #65

Task Checklist

• [ ] Have unittests been added (testing individual functions and their edge cases)
• [ ] Have integration tests been added (if applicable, to test modules of functionality)
• [ ] Have new dependencies been added to either requirements.txt, requirements_dev.txt, or docs/requirements.txt?

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📚 Documentation preview 📚: https://caftoolkit--156.org.readthedocs.build/en/156/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.01%. Comparing base (93c3ab8) to head (6b0f7af). Report is 11 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #156 +/- ## ======================================= Coverage 90.01% 90.01% ======================================= Files 17 17 Lines 941 941 ======================================= Hits 847 847 Misses 94 94 ```

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