Added initialization of spanning tree outside build routine.  This allows a extension of SpanningTree to, for instance, a DirectedSpanningTree where only the buildSpanningTree(Node root) method needs to be overridden.  As it was, buildSpanningTree contained calls to super.setRoot() and if SpanningTree was overridden, there was no way to set root anymore.  Now initialize(Root) can be called.  Didn't want to allow access to SpanningTree.setRoot() as that API should not be used for spanning trees.
Added initialization of spanning tree outside build routine.  This allows a extension of SpanningTree to, for instance, a DirectedSpanningTree where only the buildSpanningTree(Node root) method needs to be overridden.  As it was, buildSpanningTree contained calls to super.setRoot() and if SpanningTree was overridden, there was no way to set root anymore.  Now initialize(Root) can be called.  Didn't want to allow access to SpanningTree.setRoot() as that API should not be used for spanning trees.

mccraigmccraig/prefuse,mccraigmccraig/prefuse