Project & Anonymizer Model file portability across platforms

[mdevans]

Both Project and Anonymizer data files in pickle format contain pathlib.Path instances which results in PosixPath on MacOS and WindowsPath on Windows

This means a file created on Windows will not open on MacOS or Linux and visa versa.

Solution is to ensure pathlib.Path objects are not serialized to the model pickle files but instead simple strings are.

This change will need to be made on a major version change because it will break backward model file compability.