[GRASS5] s.univar + s.windavg + new code = s.cellstats

[Eric G. Miller]

It came up on the other list about calculating s.univar stats
for sites in a cell.  I've been going back and forth with Gordon Keith
on the other list and privately, about a modified s.univar.

Anyway, I've got most of a version but I hit a snag when the count
for a cell is one.  Some of the stats come up NaN.  At the moment,
I've hooked in modifications so the offending stats don't divide
by zero (which was the problem).  Now, if there's just one data point,
what are appropriate values for:

  standard deviation (0.0 ?)
  skewness (0.0 ?)
  kurtosis (-3.0?)
  coefficient of variation (0.0?)

The results above are what I get when I guard for the divide by zero
cases (n = 1).  Zero seems right for most, but I don't know about
kurtosis. It has a -3.0 bias added to it at the end.  

Once I get this resolved, I just have to hook in a proper "main",
and I'll put it up for testing...

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