[GRASS5] rst updates

[Helena]

I have added several updates for rst, including crossvalidation option
It is not finished yet, but almost everything runs OK, except larger (500K +) vector
point files with z stored as attribute (if you store the variable that you want to
interpolate as z coordinate - layer=0 it runs fine). Crossvalidation needs some speed-up too
and I plan to add a less accurate but faster and probably sufficient approximation in near future.
I would like to remove the confusing dmax option (it adds points on contours if the given
points are farther appart than dmax) - please let me know if anybody needs it (I will
then keep it and try to better explain it).

Please let me know if something that I did not mention does not work,

Helena