Use FieldTraits to access the real_type

This works even if the VectorType is a MultiTypeBlockVector.

dune/solvers/solvers/iterativesolver.hh

@@ -13,10 +13,8 @@
     template <class VectorType, class BitVectorType = Dune::BitSetVector<VectorType::block_type::dimension> >
     class IterativeSolver : public Solver
     {
-        typedef typename VectorType::value_type::field_type field_type;
-
-        // For complex-valued data
-        typedef typename Dune::FieldTraits<field_type>::real_type real_type;
+        // For norms and convergence rates
+        typedef typename Dune::FieldTraits<VectorType>::real_type real_type;
 
     public: