Added documentation to ImageExporters and ExportDescription. Moved...

Added documentation to ImageExporters and ExportDescription.  Moved GetInputMinimum() and GetInputMaximum() methods from the header to the cpp of ProcessExport.  Added sanity check to make sure the index provided by the user is within bounds and non-negative.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4169 41f8697f-d340-4b68-9986-7bafba869bb8