Added new optional optimization POLYGON which prunes points that fall outside...

Added new optional optimization POLYGON which prunes points that fall outside the image polygon of the current image being added.  Preliminary results indicate orders of magnitude of performance gains on larger networks, going from days of running time to hours.  These changes reference issue #308.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2975 41f8697f-d340-4b68-9986-7bafba869bb8