Modified grouping.  Instead of using one of the nodes in the group for the group node, creates an dummy group node which has the label of the group as its name.  Instead of using the keyset of the node coordinates added an additional variable to store the nodes associates with cluster.
Fixed bug #39 added catch statement to catch null pointer when trying to delete annotations when there are none (null pointer originates from AnnotationManager.getAnnotations which probably should return null as opposed to throwing an exception