In shadowtau, removed the "NONE" option from the "ATMNAME" parameter because...

In shadowtau, removed the "NONE" option from the "ATMNAME" parameter because an atmospheric model is required for calculating tau. Fixes #1313.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5153 41f8697f-d340-4b68-9986-7bafba869bb8