Added auto extension to isis2std and modified documentation so that the user...

Added auto extension to isis2std and modified documentation so that the user knows that world files will be generated if the cube has mapping labels. Fixes #970

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6899 41f8697f-d340-4b68-9986-7bafba869bb8