Function computes average scattering for clusters.
1 
data 

clust 
integer 
dist 
choosen metric: "euclidean" (default value), "manhattan", "correlation" 
Let scatter for set X assigned as sigma(X) be defined as vector of variances computed for particular dimensions. Average scattering for clusters is defined as:
Scatt
= (1/C) * sum{forall i in 1:C} sigma(Ci)/sigma(X)
where:
C   number of clusters, 
i   cluster id, 
Ci   cluster with id 'i', 
X   set with all objects, 
x   sqrt(x*x'). 
Standard deviation is defined as:
stdev
= (1/C) * sqrt( sum{forall i in 1:C} sigma(Ci) )
As result list
with three values is returned.
Scatt   average scattering for clusters value, 
stdev   standard deviation value, 
cluster.center   numeric matrix where columns
correspond to variables and rows to cluster centers.

Lukasz Nieweglowski
M. Haldiki, Y. Batistakis, M. Vazirgiannis On Clustering Validation Techniques, http://citeseer.ist.psu.edu/513619.html
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