Make getIterationStep return std::shared_ptr instead of &

This allows to avoid casting trickery in the calling code.

dune/solvers/solvers/iterativesolver.hh

@@ -88,15 +88,15 @@ namespace Dune {
         }
 
         /** \brief Get iteration step */
-        const ItStep& getIterationStep() const
+        const std::shared_ptr<ItStep> getIterationStep() const
         {
-          return *iterationStep_;
+          return iterationStep_;
         }
 
         /** \brief Get iteration step */
-        ItStep& getIterationStep()
+        std::shared_ptr<ItStep> getIterationStep()
         {
-          return *iterationStep_;
+          return iterationStep_;
         }
 
         /** \brief Set the error norm */