Fixes #642 - Modified low level photometric classes so they can now calculate...

Fixes #642 - Modified low level photometric classes so they can now calculate the shadow brightness value needed for estimating the optical depth (tau)

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@3829 41f8697f-d340-4b68-9986-7bafba869bb8