message: add compute_norm1_impl virtual member
diff: @@ -975,6 +975,14 @@ protected:
  */
  virtual void compute_norm2_impl(LinOp* result) const;
  + /**
  + * @copydoc compute_norm1(LinOp *) const
  + *
  + * @note Other implementations of dense should override this function
  + * instead of compute_norm1(LinOp *result).
  + */
  + virtual void compute_norm1_impl(LinOp *result) const;
  +
  /**
  * @copydoc create_submatrix(const span, const span, const size_type)
  *
  