Added a new program to Isis: shadow. This program is a higher precision...

Added a new program to Isis: shadow. This program is a higher precision version of shade, with shadow casting computations added. Fixes #1232.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5034 41f8697f-d340-4b68-9986-7bafba869bb8