Fix/pre commit isort

See merge request detectors/pycalibration!434

 The file was heavily flaked, no other changes of logic

Fix missed mixed line endings

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.