spatial conditions - add sub to match single element

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sp_match_text can be used to match an element, but does not support 
optimisations. It would also be risky to optimise it, as users could be 
matching against randomly generated or otherwise inconsistent coord_ids.

A sub such as sp_select_element() could specify optimisations.  One could argue 
the element chosen could be arbitrary, but this is semantically an unusual use 
case due to the "select" in the sub name.  

sp_select_element can just wrap around sp_match_text.

Original issue reported on code.google.com by shawnlaffan on 12 Aug 2012 at 12:16