[GRASS5] new r3.out.vtk export module

[Soeren Gebbert]

Dear developers,
i have developed a little module to export g3d voxeldata
to the vtk dataformat called "r3.out.vtk". You can select multiple
3d-rastermaps which will be written in one vtk file. To
visualize the data, you can use Paraview (www.paraview.org)
or MayaVi (http://mayavi.sourceforge.net).

I developed it with grass-6.0.0beta2, so i dont know if it is working with
older versions. (but the code is pinched from r3.out.ascii, so it should 
be work ;) )

get the r3.out.vtk here:
http://www-pool.math.tu-berlin.de/~soeren/grass/modules/

its a tar.gz archive, just extract it in the raster3d directory and add 
this dir
to the Makefile. make and make install should build it and generate the 
doku.
I have included a little description, which should be rewritten,
because my english is so horrible.

r3.out.vtk generates a structured grid (vtk dataformat) with celldata 
(data from grass).
So if you want to create isofurfaces with paraview, you have to convert
the celldata to pointdata.

I hope it is usefull. Any comment s or hints are very welcome.

Best regards
Soeren Gebbert