Only store pointers to const transfer operators

There's no need to have non-const transfer operators in the
multigrid step

dune/solvers/iterationsteps/multigridstep.hh

@@ -227,7 +227,7 @@ namespace Dune {
         std::vector<VectorType> rhsHierarchy_;
 
         //protected:
-        std::vector<MultigridTransfer<VectorType, BitVectorType, MatrixType>* > mgTransfer_;
+        std::vector<const MultigridTransfer<VectorType, BitVectorType, MatrixType>* > mgTransfer_;
 
     protected: