variance is inaccurate for arrays of identical, large values (Trac #1098)

_Original ticket http://projects.scipy.org/numpy/ticket/1098 on 2009-04-29 by @thouis, assigned to @charris._

Variance calculation is inaccurate for arrays of large, identical values:

> > > from numpy import *
> > > (ones(100000)_10.0_*20).var()
> > > 52756253943791624.0

There are more accurate algorithms for computing variance.  One example from Welford (1962) is in the attached file.
