Fixed bug in the calculation of the local normal vector for DEMs with...

Fixed bug in the calculation of the local normal vector for DEMs with non-equatorial cylindrical projections. Fixes #2243

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6171 41f8697f-d340-4b68-9986-7bafba869bb8