vtkUnstructuredGridQuadricDecimation is a filter that decimates pure
tetrahedral meshes. This version is minimally modified from its original
implementation by H. T. Vo, which is described in "Streaming Simplification
of Tetrahedral Meshes" by H. T. Vo, S. P. Callahan, P. Lindstrom, V.
Pascucci and C. T. Silva, IEEE Transactions on Visualization and Computer
Graphics, 13(1):145-155, 2007.