Matrices - add spatial export options

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Matrices with a basedata_ref, such as are generated in Cluster and RegionGrower 
analyses, should support export to spatial formats.

This can involve a to_basestruct method, followed by export of that basestruct 
in the usual way.  

An additional helper will be to add a definition query to the export, such that 
the user can define the subset of matrix rows/columns that are to be exported.  
By default it will do all rows/columns of any element that satisfies the def 
query, essentially exporting the full turnover surfaces for a set of index 
elements defined by the def query.  A second def query could be used to define 
subsets of the turnover surfaces (i.e. all within a radius of the index 
element, some random subset, etc).

Original issue reported on code.google.com by shawnlaffan on 25 Feb 2012 at 5:26