ENH: Added circle packing layout classes to VTK.
The vtkCirclePackLayout filter is similar to the area layout filter in Infovis and produces a triple for each vertex in an input vtkTree. The triple represents the (X, Y, Radius) of the circle for that vertex. There is a concrete implementation of the vtkCirclePackLayoutStrategy class for a "front chain" based circle packing layout. There is also a regression test for the layout classes and a poly data mapper class for the packed circle layout.