Added complete unittest to ExportDescription. Also changed addChannel()...

Added complete unittest to ExportDescription.  Also changed addChannel() methods to return the index of the element added in the list.  This way, applications like isis2std can use this encapsulation of indices to programatically determine the index of the red, green, blue, or alpha channel without needing to hardcode it.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4170 41f8697f-d340-4b68-9986-7bafba869bb8