Allow l-values as arguments

dune/solvers/iterationsteps/blockgssteps.hh

@@ -310,9 +310,10 @@ auto gs(double tol = LinearSolvers::defaultGsTol,
 template <class Matrix, class Vector, class BitVector, class LocalSolver>
 struct BlockGSStep : public LinearIterationStep<Matrix, Vector, BitVector> {
 
-  BlockGSStep(LocalSolver&& localSolver,
+  template<class LS>
+  BlockGSStep(LS&& localSolver,
               BlockGS::Direction direction = BlockGS::Direction::FORWARD)
-      : localSolver_(localSolver)
+      : localSolver_(std::forward<LS>(localSolver))
       , direction_(direction) {}
 
   void iterate() {