We cannot rely on the directory creation date, as it might get modified with filesystem upgrades. File modification dates are conserved though (not creation date), so we must use these when the information is not available from run metadata

This metadata file subsumes the old retrieved constants YAML file.  The commit includes some refactoring centered around pathlib.

Index error means that there are no files in the directory. We catch this error, as we want get_dir_creation_date to raise only a ValueError, which highlights better that the provided run is incorrect