Add unit test for HornNormal functionality
 - Add new unit test based on fabricated input image
 - Generated golden/baseline output images for various tests and verified the results
 - Add functionality for comparing two rendered images that use different values for
   nodata by translating nodata values to different values that can be easily compared.
Merge branch 'master' of github.com:ngageoint/mrgeo
Merge HornNormal unit test changes