Clustering - allow calculations per node as a separate process after clustering has been completed

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The system supports the calculation of indices for each cluster node, 
essentially treating the elements associated with each node to be a 
non-geometric spatial window.  

Currently this process is integrated with the cluster analysis, so if one 
changes one's mind about which indices are to be calculated then the entire 
cluster analysis must be re-run.  

Allowing this as a separate process will make life simpler, particularly when 
the cluster analyses take extremely long periods of time.  

Original issue reported on code.google.com by shawnlaffan on 1 May 2012 at 3:27