Use std::shared_ptr instead of Dune::shared_ptr

dune/tnnmg/functionals/sumfunctional.hh

 #ifndef DUNE_TNNMG_FUNCTIONALS_SUMFUNCTIONAL_HH
 #define DUNE_TNNMG_FUNCTIONALS_SUMFUNCTIONAL_HH
 
+#include <memory>
+
 #include "dune/tnnmg/functionals/nonsmoothconvexfunctional.hh"
 
 namespace Dune {
@@ -25,7 +27,7 @@ namespace Dune {
         psi_ = Dune::stackobject_to_shared_ptr(psi);
       }
 
-      SumFunctional(Dune::shared_ptr<NonlinearityType>& phi, Dune::shared_ptr<NonlinearityType>& psi) :
+      SumFunctional(std::shared_ptr<NonlinearityType>& phi, std::shared_ptr<NonlinearityType>& psi) :
       phi_(phi),
       psi_(psi)
       {}
@@ -115,8 +117,8 @@ namespace Dune {
       }
 
     private:
-      Dune::shared_ptr<NonlinearityType> phi_;
-      Dune::shared_ptr<NonlinearityType> psi_;
+      std::shared_ptr<NonlinearityType> phi_;
+      std::shared_ptr<NonlinearityType> psi_;
     };
 
   }   // namespace TNNMG