Refactor TethysWorkspace to always return Path objects

[shawncrawley]

The TethysWorkspace class can be initialized by a pathlib.Path object, and stores the path as a pathlib.Path object internally, but all methods that return a path do so as a string, not the object itself.

I think we held off on doing this refactor before because it would be a breaking change for app developers. But I think it's worth doing. It's definitely more of a pain to use in coding contexts where someone is all-in on pathlib.Path. It's easier for someone who wants the strings to just recast the returned object (e.g. str(workspace.path) rather than just workspace.path).

[swainn]

Yes, I believe that was why it was implement the way it is--backward compatibility.