Merge branch 'fix/error-norm' into 'master'

Fix return type in IterativeSolver

See merge request !60

dune/solvers/solvers/iterativesolver.hh

@@ -107,7 +107,7 @@ namespace Dune {
         }
 
         /** \brief Get error norm */
-        const ItStep& getErrorNorm() const
+        const ErrorNorm& getErrorNorm() const
         {
           return *errorNorm_;
         }