Adapt to upstream dune-solvers changes

dune/tnnmg/iterationsteps/tnnmgstep.hh

@@ -152,8 +152,8 @@ public:
     auto iterativeSolver = std::dynamic_pointer_cast<Solvers::IterativeSolver<typename Linearization::ConstrainedVector> >(linearSolver_);
     if (iterativeSolver)
     {
-      iterativeSolver->iterationStep_->setIgnore(emptyIgnore);
-      linearIterationStep = dynamic_cast<LinearIterationStepType*>(iterativeSolver->iterationStep_);
+      iterativeSolver->getIterationStep().setIgnore(emptyIgnore);
+      linearIterationStep = dynamic_cast<LinearIterationStepType*>(&(iterativeSolver->getIterationStep()));
     } else
       DUNE_THROW(Exception, "Linear solver has to be an IterativeSolver!");