Spatial conditions - use pre_calc and related methods like Biodiverse::Indices

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The spatial conditions are getting complex when there is caching involved.  It 
would be much simpler to generalise the approach used for Biodiverse::Indices, 
where pre_calc methods are specified for each sub and called as needed.  

The caches can then just be passed as args from prec_calc_global method calls, 
with cleanup being via post_calc and post_calc_global calls.  

Original issue reported on code.google.com by shawnlaffan on 17 Jun 2012 at 5:21

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Original comment by shawnlaffan on 14 Nov 2013 at 4:28

• Changed title: Spatial conditions - use pre_calc and related methods like Biodiverse::Indices
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