[GRASS5] v.surf.idw new parameter power

[Hamish]

> I needed to be able to modify the power of the distance part of the
> idw algorithm (the beta). Well I did it, and I've attached the diff.

often refered to as the "coefficient of friction", 1/d^Cf 

A value of 2 gives the smoothest result, but values greater than two
will help to constrain open boundary effects. A compromise is to run at
2.5 or run the module twice, once for Cf=2 and once for Cf=3. Then use
r.series or r.mapcalc to average the results. Values at real data points
should be the same, only the interpolated values are tweaked, so legal.

For giggles, other radial basis functions can be inserted too. (log() etc)
 

Hamish
(I have an interesting paper to publish on this at some point)