[vtkusers] A new way to create tetrahedra

[Samson Timoner]

I've added the class vtkRectiliniearGridToTetrahedra to VTK. This
class takes in a RectilinearGrid and divides each cell into 5, 12, 6
or combinations of 5 and 12 tetrahedra.

VTK now has two methods to generate tetrahedra automatically. The
other is Delaunay Tetrahedralization.

In general, there are roughly 3 ways to create tetrahedra:
 -- Choose a set a points and use Delaunay Tetrahedralization
 -- OctTree methods: create a grid, subdivide into Tetrahedra,
        and then recursively subdivide.
 -- Advancing Front Methods: Add points around the boundaries of
        existing tetrahedral mesh and grow the mesh.

I have checked in a rather simple OctTree based method.

I have written far more complicated methods and I might be
willing to rewrite them into VTK, but I do not know of a good way to
handle trees of tetrahedra in VTK. Such trees are critical to
effecient and effective tetrahedral based methods. If someone has a
suggestion as to how to handle trees of tetrahedra in VTK, please let
me know. Otherwise, after I publish a few papers, I will release my
60,000 line tetrahedra mesh library to the public.

 Cheers,

-- Samson

P.S. If you wish more information on generating and manipulating tetrahedra,
     see Chapter 2 of my Ph.D. thesis (and the references therein) at 
     http://www.ai.mit.edu/people/samson/papers/publications.html. The pdf
     file is hyperlinked directly to many of the referenced papers.

P.P.S. Special thanks to Mike Halle and was tremendously helpful in submitting
       my class to VTK.