BUG: The recent modification attempting to run labelfusion only on output images that get created broke the script
in parallel environments. The check for existing output takes place immediately after job submission, when no
output exists.

Also, there is no record of how many atlases were used, unless -k 1 was specified. When using the default options,
the user cannot know how many atlases actually got warped to the subject space. Most of the atlas registration jobs
could fail, or you might have twice as many atlases in one target as the next, and you would not be able to tell.