[GRASS5] s.mask added

[Eric G. Miller]

I added a module called "s.mask" to CVS to filter sites lists via a
raster "mask".  This flowed out of a request/discussion with Helen
Mitasova.  It's not very tricky, but should help folks who just want to
display/analyze sites that "intersect" with non-NULL cell values in a
raster.  It can use the current region or the raster's cell head,
depending on which the user thinks is more appropriate.

Ciao,
-- 
Eric G. Miller <egm2 at jps.net>