new program: s.windavg

[Darrell McCauley]

I just wrote a quick program to average sites inside areas defined by
cell boundaries. Output is the area center's easting, northing, and
the average.  It's got an option to put the output in g.gnuplot format
for quick visualization of spatial trends in sites lists (see the man
page, included with source).

If anyone has any use for this, it's on pasture.ecn.purdue.edu
(128.46.161.85, 128.46.133.85, 128.46.175.85) in
pub/mccauley/grass/s.windavg.tar.gz

--Darrell