Indices - endemism - use group area property in range calculations if set

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The endemism calculations currently use the label range property if it is set.  
However, they should also use an area (or range) property if set for the 
groups.  This would allow the use of irregular geographic units in an analysis 
but still represented by, for example, polygon centroids.  Otherwise in such 
cases the ranges are calculated from the number of centroids.  

This needs to propagate through the full suite of endemism calculations 
including PE.  

Note that the area property needs to be set for all groups for this to be 
sensible, as there is no sensible default.  All or nothing can be a 
precondition for the calculations.  

Original issue reported on code.google.com by shawnlaffan on 3 Aug 2014 at 8:46