storing BIDSPath.root attribute as a string instead of Path?

[sappelhoff]

Came up in #831 (comment)

We had problems across platforms: On POSIX, BIDSPath.root has forward slashes when printed (__str__), on Windows, it has backslashes. That's undesirable for testing, because BIDS wants forward slashes-only, and it's not consistent anyhow.

Richard had the idea to store BIDSPath.root as a string, and only convert it back to Path when needed.

It would be best if we could store it as a POSIX string ... and then upon converting back to Path, on POSIX it would be a POSIX-Path, whereas on Windows it would be a Windows-Path.

We haven't tested it though.