spatial conditions - add sub for point in polygon test from shapefile

[GoogleCodeExporter]

This would allow easier specification of complex polygons than 
sp_point_in_poly() (issue #221).  

e.g.:
sp_point_in_shape_poly (polygon => 'some_file.shp', point => [0.5,0.5])


Caching of the polygon would be needed for performance reasons, as well as 
splitting of multipart polygons into individual polys.

The addition of an axes argument would be useful to allow multi-dimensional 
groups will need to be assessed against shapefiles with fewer dimensions.  

Original issue reported on code.google.com by shawnlaffan on 29 Mar 2011 at 12:00