Refactored auto gunzip to work-around fsl glitches

Apparently, fsl thinks it's smarter than it's users and tries to open
more files that it's ased to open. E.g., if you tell it to open
img.nii.gz, it will also try to open img.nii. So, it got confused by
our nice auto-gunzip feature, which would make both img.nii.gz and
img.nii available. So, until fsl is better behaved, we'll have
work-around by typing img.nii.ugz to get to the gunzipped file.