'bugfix' for s.surf.idw

[Markus Neteler]

> I checked it and it seems that there is a mistake in the calculation of
> the cells. If I remove the '+0.5' from the formule 'cell=sum1/sum2 + 0.5'
> (somewhere around line 196 in main.c) then it gives the correct results
> for my data. I think that the '+0.5 was used tho round integers in a
> correct way, but because it works with floats now it isn't necessary
> anymore.

Of course! I have seen it after sending the message.

Thanks for the info

 Markus