[vtkusers] Extract cells from a 2D background grid based on overlay grid

[Karl Merkley]

I would like to extract a set of cells from a background mesh based on  
the geometry of an overlaid grid.    I think I should be able to this  
by using a vtkOBBTree to create the bounding area, creating a  
vtkImplicitSelectionLoop from the set of oriented bounding boxes and  
then using the selection loop in a vtkExtractGeometry filter.     
However,  I don't know how to create the selection loop from the  
bounding boxes.

Am I on the right track?    Any ideas on how to go from the OBBTree to  
the ImplcitSelectionLoop?

    Thanks,
          Karl