MultiGridSet: add overload to set pre- and postsmoother individually using `shared_ptr`

This matches the overloads to set both pre- and postsmoother to a
common smoother.

dune/solvers/iterationsteps/multigridstep.hh

@@ -178,6 +178,15 @@ namespace Dune {
                 levelWiseSmoothers_.clear();
         }
 
+        virtual void setSmoother(std::shared_ptr<LinearStepType> preSmoother,
+                                 std::shared_ptr<LinearStepType> postSmoother)
+        {
+                presmootherDefault_  = std::move(preSmoother);
+                postsmootherDefault_ = std::move(postSmoother);
+
+                levelWiseSmoothers_.clear();
+        }
+
         /** \brief Set the smoother iteration step for a particular level */
         virtual void setSmoother(LinearStepType* smoother, std::size_t level)
         {