[yt-dev] Issue #639: PlotWindow save should accept directory prefix (yt_analysis/yt)

New issue 639: PlotWindow save should accept directory prefix https://bitbucket.org/yt_analysis/yt/issue/639/plotwindow-save-should-accept... Matthew Turk: PlotWindow's save should detect if the filename supplied ends with `os.pathsep` and if so, it should simply prefix that before the filename it would otherwise generate.