Paths should be path objects and not just concatenated strings

[dnowacki-usgs]

Currently we write paths as something like "your/path/goes/here".

And then modify paths by doing something like

    basefile = metadata["basefile"]

    if "prefix" in metadata:
        basefile = metadata["prefix"] + basefile

Those are string paths that we are concatenating using the addition operator.

This is not ideal, since it may be confusing to Windows users (who are used to seeing backslash-separated paths), the current method is not obvious, and because Python provides a more robust method of dealing with paths. We should consider using os.path or probably better, pathlib.