[STATSGRASS] masks

[H Bowman]

> Currentely, do masks in grass have to be
> 1 and 0, or 1 and null?

as far as I can tell, they are true for any positive value, false for
null or 0, and I'm not sure about negative values but probably true as
well.

A couple of modules still might not have been updated with support for
nulls, so may require 0/1 null maps, but I think that would be pretty
rare.

If you use r.mapcalc to make your MASK, results are pretty straight
forward.


Hamish