Add setMatrix() method to DenseMultigridTransfer

Again, see #14. This time without additional garbage, sorry.

dune/solvers/transferoperators/densemultigridtransfer.hh

@@ -121,6 +121,11 @@ public:
         return matrix_;
     }
 
+    /** \brief Set matrix */
+    template <class M>
+    void setMatrix(M&& m) {
+        matrix_ = std::forward<M>(m);
+    }
 
 protected: