Matrix building - exclude empty groups

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What steps will reproduce the problem?
1.  Run a cluster analysis with a basedata containing empty groups.

What is the expected output? What do you see instead?

The resultant cluster tree contains empty groups.  These should in reality be 
undefined (in terms of clustering and related analyses).  

This is related to issue #228, but fixing this aspect will be more general as 
some indices can be used to cluster but are valid for spatial analyses where 
the neighbour set contains only empty groups (e.g. Simpson and Shannon indices).

In terms of a solution, there are two options.
  1.  Add a condition to the definition query for each cluster analysis (either exposed or implicit).
  2.  Add an option to Biodiverse::Spatial::sp_calc to exclude empty groups from the elements to calc.

The latter requires less coding and follows from issue #223.  

Original issue reported on code.google.com by shawnlaffan on 25 Apr 2011 at 4:54