Make norm find correct field_type when using std::vector

dune/solvers/norms/norm.hh

@@ -3,6 +3,8 @@
 #ifndef DUNE_SOLVERS_NORMS_NORM_HH
 #define DUNE_SOLVERS_NORMS_NORM_HH
 
+#include <dune/common/ftraits.hh>
+
 namespace Dune {
 namespace Solvers {
 
@@ -13,7 +15,7 @@ class Norm {
     public:
         typedef V VectorType;
 
-        typedef typename VectorType::field_type field_type;
+        using field_type =  typename Dune::FieldTraits<VectorType>::field_type;
 
         /** \brief Destructor, doing nothing */
         virtual ~Norm() {};