MAINT: add todo

include/caosdb/entity.h

@@ -203,6 +203,7 @@ private:
   public:
     explicit iterator(const RepeatedPtrFieldWrapper<T, P> *instance,
                       int index = 0);
+    // TODO(henrik) add unit tests
     auto operator*() const -> T &;
     auto operator++() -> iterator &;
     auto operator++(int) -> iterator;