tuples of graph's node table as node proxy tuples

If a graph's or tree's tuples are accessed they are returned as
TableTuple but should be TablesNodes, so that expressions like
TREEDEPTH() work (cp. #34).

src/prefuse/data/Graph.java

@@ -281,8 +281,10 @@ protected void init(Table nodes, Table edges, boolean directed,
         }
 
         // set up tuple manager
-        if ( m_nodeTuples == null )
+        if ( m_nodeTuples == null ) {
             m_nodeTuples = new TupleManager(nodes, this, TableNode.class);
+            nodes.setTupleManager(m_nodeTuples);
+        }
         m_edgeTuples = new TupleManager(edges, this, TableEdge.class);
 
         // set up node attribute optimization