[bugfix] Adapt nonoverlapping computations to changes in UG

dune-grid has introduced a distinction between the two capabilities HasEntity and HasEntityIterator several years ago. So far, UG did not distinguish the two. After recent changes it correctly reflects UG's capabilities: HasEntity is true for all codims, while HasEntityIterator is not. In PDELab we used HasEntity to guard iteration, which is not a correct usage pattern anymore. The commit changes PDELab to use the HasEntityIterator capability instead.

This fixes #166 (closed)