TST: skip gsd-based tests on Windows

[tylerjereddy]

As far as I can tell our gsd dependency is not Windows compatible so I've marked the corresponding unit tests for skipping based on platform.

If we agree that gsd isn't windows compatible, there's probably also some kind of documentation or warning related to import that might be suitable.

[tylerjereddy]

I noticed tracebacks in the gsd source that used i.e., a / explicitly in a path setting code instead of something like os.path.join(), so the compatibility fixes on their end might not be that difficult, but I think is outside our remit beyond possibly raising an issue with them if possible / appropriate.

[orbeckst]

Please raise an issue with gsd.

But I think that instead of skipping GSD tests per se, we should have something in our GSD reader that fails gracefully under Windows and then test that it does fail nicely.

EDIT: I guess you'll still need the "skip GSD tests under NT" but this still leaves the main code open to a bad user experience with GSD under Windows.

[tylerjereddy]

I've opened the issue on their bitbucket in any case.