[GRASSLIST:10505] 3d vector export

[Sören Gebbert]

Hi,
i have updated "v.out.vtk" to support "layer" category selection
and made a screenshot, showing  grass 3d vectors 
(exported with v.out.vtk) within Paraview from the spearfish60 location.

http://www-pool.math.tu-berlin.de/~soeren/grass/modules/screenshots/v.out.vtk-r.out.vtk-comb.png

The 3d vectors are are 2d vectors (roads, streams and bugsites)
from spearfsih60 which has been modified with "v.drape".

v.out.vtk is available at:
http://www-pool.math.tu-berlin.de/~soeren/grass/modules/

Best regards
Soeren