ENH: Newly refactored vtkGraph and vtkTree. Among the major changes are:

* Iterator-based access to edges
* vtkGraph subclasses from vtkDataObject instead of vtkPointSet
* vtkGraph's implementation is much more like that of the boost graph library
* Separate classes vtkDirectedGraph and vtkUndirectedGraph instead of
a Directed flag
* Mutable classes vtkMutableDirectedGraph and vtkMutableUndirectedGraph used for building graphs