add support for filepaths in compare

[gdementen]

I relatively often try to use compare with filenames (containing sessions) but it does not work.

I have to write:

compare(Session(path1), Session(path2), names=[path1.stem, path2.stem])

and I wonder if it would be worth it to implement support for str and Path objects. Honestly, this would probably not be worth it if we had #228, but this is so much easier to implement that it might be worth it as a stopgap solution.

[gdementen]

Given that view() and edit() support filepaths, I see no reason not to implement this in compare().