message: Make Line::length const
diff: @@ -36,7 +36,7 @@ class Line_t : public Shape<bg::model::linestring<T>, T> {
  return Shape<bg::model::linestring<T>, T>::add_point(p) && recompute_s();
  }
  - auto length() { return bg::length(Shape<bg::model::linestring<T>, T>::obj_);}
  + auto length() const { return bg::length(Shape<bg::model::linestring<T>, T>::obj_);}
  int size() { return Shape<bg::model::linestring<T>, T>::obj_.size(); }
  