Prevent loading of duplicate files into the same capture

[poynting]

If you try to load an imageset with two folders having copies of the same file, the data will combine successfully but will result in downstream issues due to 'extra' band/dls information returned by some methods of Capture.

I think the right thing to do here is to check the UUID of the image when adding it to a Capture object to prevent loading duplicates, even duplicates with different file names. Alternatively we could use the current Image object eq compare, but this should probably be updated to use the UUID as well.