spatial analyses - add optimisation for cases where nbr sets are always the same

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The sp_point_in_poly() and sp_point_in_poly_shape() subs both return the same 
set of neighbours for a given input.  In such cases we can cache the set of 
groups that occur in the polygons and access them directly in subsequent calls. 
 This avoids all the overheads of searching incurred when setting up the 
neighbour search.  

The risk is that some cases will not always be the same.  Need to allow the 
user to turn off the optimisations.  

Original issue reported on code.google.com by shawnlaffan on 25 Jun 2012 at 11:48