[Dune] CurvilinearGridAPI: Implementation of EntityTags and BoundaryIterators

[Bernd Flemisch]

Hi, 

1. What I like most about Dune is a clear separation of concepts. And I see no need to violate this here, since the mentioned possibilities exist. As Christoph said, we also use GridPtr a lot. What would be so wrong with generalizing it such that it can also treat gmsh? (and maybe give it a better name?) 

2. A grid has the method hasBoundaryIntersections which gets an element. If you put that before the loop over the intersections, you at least get the impression of increasing efficiency. 

Kind regards 
Bernd