Frame export

We would like to add the functionality to export frames as multipage tiff files. Theses files can be consumed by ImageJ (automatically imported as image stacks) and also by Gwyddion (here you have the option during the import, to import as images or as volume data). This would help further analyzing, e.g., slicing through frames and observe pixel value changes as a function of time. Another advantage of multipage tiff is, that you can preserve accuracy because single precision floats are supported.

Further, I would like to add support for Gwyddion's .gwy files, right now only .gsf is supported which only allow one frame per file and no metadata can be added. In .gwy we could have an entire movie (or slices) as one file and all metadata can be obtained with Gwyddion's metadata-browser.

I think we could add this to the export_frame method of the FastMovie class, just by allowing "tiff" and "gwy" as file_format arguments. I just would like to refactor this method, as it hard to wrap your head around right now (~140 lines, deeply nested if-branches).

Please let me know if you have any strong opinions about this.