Parallel/vtkPBGLCollectGraph.cxx:
  - Simplify CopyStructureOfDataSetAttributes

Parallel/vtkPBGLCollapseGraph.cxx,
Parallel/vtkPBGLCollapseGraph.h,
Parallel/Testing/Cxx/TestPBGLCollapseGraph.cxx:
  - New filter that collapses a graph based on commonalities in the value of
    a vertex property (e.g., collapse vertices with the same component number).