Use global index sets by the dune-parmg module

This MR contains most of the changes of !22 (closed) , with some extra cleanup.

dune/gfe/parallel/globalp2mapper.hh

@@ -11,7 +11,7 @@ namespace Dune {
   class GlobalP2Mapper : public Dune::GlobalMapper<Basis>
 
   {
-    using typename Basis::GridView;
+    using GridView = typename Basis::GridView;
     using P2BasisMapper = Functions::LagrangeBasis<GridView,2>;
   public:
     /** \brief The integer number type used for indices */