multigridstep.cc: use `std::make_shared`

dune/solvers/iterationsteps/multigridstep.cc

@@ -119,8 +119,8 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::preprocess()
     // /////////////////////////////////////////////////////
     for (int i=this->numLevels()-2; i>=0; i--)
     {
-        this->matrixHierarchy_[i] = std::shared_ptr<MatrixType>(new MatrixType);
-        this->xHierarchy_[i] = std::shared_ptr<VectorType>(new VectorType);
+        this->matrixHierarchy_[i] = std::make_shared<MatrixType>();
+        this->xHierarchy_[i] = std::make_shared<VectorType>();
 
         // Compute which entries are present in the (sparse) coarse grid stiffness
         // matrices.