Move baseline images to an external repository.

1) Baseline images are now stored in the github UV-CDAT/uvcdat-testdata
   repository.
2) A new build target, update-testdata, is added. This will clone the
   repository to a customizable location ([build_dir]/uvcdat-testdata by
   default). The location can be shared between multiple uvcdat clones.
   If the repository already exists, is on master, and is clean, it will
   be updated when this target is executed.
3) Updated testing scripts to use new baseline location.