Added support for GIFs. Fixes #5

In order to support all basic image formats, we are adding support for `GIF`. This includes support for animated GIFs, meaning 4d arrays. I chose to use checkerboards for the patterns since GIFs have a limited color palette and we have already tested monochrome upcasting in other cases.

In this PR:

- Added test for unanimated/animated GIFs
- Added support for unanimated/animated GIFs