vtkMutableDirected/UndirectedGraph now have RemoveVertex,
RemoveEdge functions. Each re-assigns (moves) the last item
in place of the deleted item. RemoveVertices and RemoveEdges
are also available for removing sets of vertices and edges.

ENH: Adding vtkStreamGraph

vtkStreamGraph continuously merges its input with an internal graph,
and returns that internal graph.

ENH: Adding remove capability to mutable graph helper.

ENH: Adding capability to limit the number of edges in vtkMergeGraph and vtkStreamGraph.

ENH: Adding fast b-spline option to vtkSplineGraphEdges.

ENH: Adding capability for graph pipeline inside a vtkTreeAreaView to report progress from its internal filters.

This required making a RegisterProgress() function on
vtkHierarchicalGraphPipeline.

STYLE: Improving documentation for vtkMergeGraphs, vtkStreamGraph.