[Dune::AtType] Bugfix: member "Type" should be a type, not a variable!

Also avoid use of deprecated Dune::ElementType in favor of tuple_element.

[[Imported from SVN: r6070]]

dune/common/utility.hh

@@ -485,8 +485,8 @@ namespace Dune {
    */
   template <int N, class Tuple>
   struct AtType {
-    typename ElementType<Length<Tuple>::value - N - 1,
-        Tuple>::Type Type;
+    typedef typename tuple_element<Length<Tuple>::value - N - 1,
+        Tuple>::type Type;
   };
 
   /**