[gridfactory] provide a comm() method

dune/alugrid/3d/gridfactory.hh

@@ -65,6 +65,8 @@ namespace Dune
     //! type of matrix from world coordinates to world coordinates
     typedef typename Transformation::WorldMatrix WorldMatrix;
 
+    typedef typename Grid::CollectiveCommunication Communication;
+
   private:
     static_assert ( (elementType == tetra || elementType == hexa),
                     "ALU3dGridFactory supports only grids containing "
@@ -318,6 +320,11 @@ namespace Dune
 
     const std::vector<unsigned int>& ordering () const { return ordering_; }
 
+    Communication comm() const
+    {
+      return Communication(communicator_);
+    }
+
   private:
     unsigned int boundaryInsertionIndex ( const typename Codim< 0 >::Entity &entity, int face ) const
     {