New module for running statistics calculations

[Patrick Alken]

Hi all,

   I've just added a new module to GSL for running (or online) 
statistics - ie: computing the mean, variance, standard deviation, 
skewness, kurtosis, median, and arbitrary percentiles on the fly with a 
single pass algorithm, without needing to store the whole dataset in 
memory at once.

   The mean, variance, standard deviation, skew and kurtosis are exact 
computations. The median and p-quantile algorithm provides an 
approximation to the actual quantile, using the algorithm of:

         R. Jain and I. Chlamtac, The P^2 algorithm for dynamic 
calculation of quantiles and
         histograms without storing observations, Communications of the ACM,
         Volume 28 (October), Number 10, 1985, p. 1076-1085.

It is now in the 'master' branch on the git and documented. I'll add an 
example program a little later this week. Any feedback is welcome.

Patrick