[GRASS5] New volume data set

[Helena Mitasova]

New data set for testing the GRASS5.7 volume support has been added
to the GRASS Sample datasets (http://grass.itc.it/data.html):
it includes Slovakia DEM and Precipitation data.
You can get it directly from here:
http://mpa.itc.it/grasstutor/data_menu2nd.phtml
See sample visualization here http://grass.itc.it/grid3d/index.html
(images on the top of the page)
The volume visualization still needs some "tuning"
so below are some hints how to make the images.
                                                                                            
Let us know if there are any serious problems
(but read the below hints before complaining that you don't see
anything)
                                                                                            
Helena
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Quick tour:
 The map 'dem500' is used for a DEM.
 There is a 3D file 'precip3d.500z50' that can be used
 for isosurfaces and cross-sections.
                                                                                            
 nviz el=dem500
                                                                                            
 Then load the volume (Panel -> Volume -> New).
                                                                                            
 To see the isosurfaces use values between 600 and 1300
 (e.g. 700 and 1000 with transparency of approx. 100 and
  a third isosurface 1300 with no transparency).
                                                                                            
 Set the polygon resolution to 3 or less (note that for
 resolution 1 you may need to wait a little depending on your
 machine's speed) to render the volume.