removed make_relation_node method from repository_node

include/matador/object/attribute.hpp

@@ -6,6 +6,7 @@
 #include "matador/utils/field_attributes.hpp"
 
 #include <memory>
+#include <ostream>
 
 namespace matador::object {
 
@@ -60,6 +61,8 @@ public:
     return type() == utils::data_type_traits<Type>::type(attributes().size());
   }
 
+  friend std::ostream& operator<<(std::ostream& os, const attribute& attr);
+
 private:
   friend class object;
   friend class attribute_generator;