Added SCALE and UNITS parameters so that radiance output is supported in...

Added SCALE and UNITS parameters so that radiance output is supported in addition to I/F. Added more user documentation and comments in order to make the radiometric correction equation clearer. Fixed formatting of equations in XML description section. Added new test for radiance output.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2400 41f8697f-d340-4b68-9986-7bafba869bb8