Item classes redesigned to make them easier to use, less likely to be abused,...

Item classes redesigned to make them easier to use, less likely to be abused, and to eliminate duplicate code.  Several bugs were fixed, and several more were probably introduced.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2174 41f8697f-d340-4b68-9986-7bafba869bb8