New I/O options, realspace integration

[sezelt]

This adds:

• The ability to load raw files from the EMPAD-G2 (this functionality is enabled automatically if the empad2 package is installed)
• The ability to export the datacube or virtual images in various formats. (Note that saving of the virtual images was always somewhat possible thanks to pyqtgraph, but this gives more relevant options).
• The ability to read npy and npz files that contain a single 4D array
• iCoM virtual images
• Diffraction patterns can be summed from a region of the scan (closes Integrate over multiple pixels in real-space #2)