spatial analyses and recycling - linearise approach

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The recycling in Spatial.pm currently identifies which groups can be recycled 
into and then applies the results accordingly.  

If we ever get parallel processing going (which we should) then linearising 
this is probably going to simplify things.  

In this approach a hash of the recyclables can be built and checked as we 
process each group, rather than assigning to all groups in a recyc set and then 
skipping them when they are to be processed.  

This all assumes the hash can be shared across threads/forks.  

Original issue reported on code.google.com by shawnlaffan on 3 Oct 2014 at 6:23