demprep.xml: Fixed handling of special pixels. Now reports the min/max radius...

demprep.xml: Fixed handling of special pixels. Now reports the min/max radius encountered. A negative radius will now cause this program to fail. 

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2072 41f8697f-d340-4b68-9986-7bafba869bb8