New export

Reviewed the export, fixing many problems. Still not multi-threaded, too difficult. It should be able to export any type of loaded sequence to any type of choice, with the option of normalizing the sequence, of cropping the input to selection, of scaling images to lo/hi and resampling the images for film formats. FITS cubes are now handled as input and output.

Also working on #649 (closed), performance concerns for FITS reading.