Added edge-pruning to MinimumSpanningTree() such that measures whose points do...

Added edge-pruning to MinimumSpanningTree() such that measures whose points do not connect to another graph node are not included in the MST

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4260 41f8697f-d340-4b68-9986-7bafba869bb8