message: Make a getter function const
diff: @@ -38,7 +38,7 @@ class Line_t : public Shape<bg::model::linestring<T>, T> {
  auto length() const { return bg::length(Shape<bg::model::linestring<T>, T>::obj_);}
  - int size() { return Shape<bg::model::linestring<T>, T>::obj_.size(); }
  + int size() const { return Shape<bg::model::linestring<T>, T>::obj_.size(); }
  void append_linestring(const Line_t &ls) {
  // TODO(@fortiss): wrong
  