Fix: Persist timeindex_worst_cases in edisgo.save()

[nader-00]

Description

• Add saving of timeindex_worst_cases.csv in TimeSeries.to_csv()
• Add restoration of timeindex_worst_cases with datetime parsing in TimeSeries.from_csv()
• Update file count expectation in test_save() to include worst-case metadata file
• Add regression test coverage for timeindex_worst_cases save/load
• Remove pytest.skip() fallback in test_generator_import() as database is accessible

Fixes #351

Type of change

Please delete options that are not relevant.

• 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)

Checklist:

• New and adjusted code is formatted using the pre-commit hooks
• New and adjusted code includes type hinting now
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The Read the Docs documentation is compiling correctly
• If new packages are needed, I added them the setup.py, and if needed the rtd_requirements.txt, the eDisGo_env.yml and the eDisGo_env_dev.yml.
• I have added new features to the corresponding whatsnew file