[!303] Add specialization of ConstructionTraits for SeqGS

Merge branch 'issue/missing_gs_construction_traits' into 'master'

ref:core/dune-istl

### Summary

This MR adds a missing specialization of the template ConstructionTraits for
the sequential preconditioner SeqGS.

See merge request [core/dune-istl!303]

  [core/dune-istl!303]: gitlab.dune-project.org/core/dune-istl/merge_requests/303

dune/istl/paamg/smoother.hh

@@ -202,6 +202,22 @@ namespace Dune
       }
     };
 
+
+    /**
+     * @brief Policy for the construction of the SeqGS smoother
+     */
+    template<class M, class X, class Y, int l>
+    struct ConstructionTraits<SeqGS<M,X,Y,l> >
+    {
+      typedef DefaultConstructionArgs<SeqGS<M,X,Y,l> > Arguments;
+
+      static inline std::shared_ptr<SeqGS<M,X,Y,l>> construct(Arguments& args)
+      {
+        return std::make_shared<SeqGS<M,X,Y,l>>
+          (args.getMatrix(), args.getArgs().iterations, args.getArgs().relaxationFactor);
+      }
+    };
+
     /**
      * @brief Policy for the construction of the SeqJac smoother
      */