Feature add preprocessing unit tests

[endast]

What

This PR adds unit tests for the preprocssing pipeline, most of the test use input files and a "expected" output file that the results get compared to. It also adds a github action that runs the unit tests.

Testing

Check out the code, activate the env and run pytest.

mamba env create --file deeprvat_preprocessing_env.yml
mamba activate deeprvat_preprocess
pip install -e .
pytest -v

The tests should pass :)