Fixed an error were GroundGrid would thrown an exception when calculating...

Fixed an error were GroundGrid would thrown an exception when calculating longitude grid lines near 90 latitude. Fixes #4766.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7570 41f8697f-d340-4b68-9986-7bafba869bb8