[Bugfix] Corrects current residual after postsmoothing.

Previously the current residual was not updated after the coarse
grid correction. This patch fixes this by storing the only the
prolongated coarse grid correction in context.lhs and updating the
accumulated update. During postsmoothing the currently residual is
updated according to context.lhs.

dune/istl/paamg/twolevelmethod.hh

@@ -402,9 +402,10 @@ public:
     policy_->moveToCoarseLevel(*context.rhs);
     InverseOperatorResult res;
     coarseSolver_->apply(policy_->getCoarseLevelLhs(), policy_->getCoarseLevelRhs(), res);
-    policy_->moveToFineLevel(*context.update);
-    // Postsmoothing
     *context.lhs=0;
+    policy_->moveToFineLevel(*context.lhs);
+    *context.update += *context.lhs;
+    // Postsmoothing
     postsmooth(context, postSteps_);
 
   }