spatial conditions - allow user to override the validity checks

[GoogleCodeExporter]

The system currently checks the validity of the spatial conditions.  This makes 
it easier to debug conditions. 

However, the checks are imperfect (and actually quite basic).  At times it 
flags correct specifications as invalid when they are valid.  This is more when 
there are complex specifications.  

It would be useful to allow users to override the system, with a prominent 
caveat emptor being shown.  

Original issue reported on code.google.com by shawnlaffan on 17 Jun 2012 at 5:08