Added methods in tree item classes for detecting points, measures, and nodes...

Added methods in tree item classes for detecting points, measures, and nodes within subtrees; First commit for AbstractFilter, AbstractPointFilter, and PointIdFilter classes

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2296 41f8697f-d340-4b68-9986-7bafba869bb8