preparations for object class changes

include/matador/object/constraint.hpp

@@ -3,6 +3,7 @@
 
 #include "matador/utils/constraints.hpp"
 
+#include <memory>
 #include <ostream>
 #include <string>
 #include <variant>
@@ -57,6 +58,7 @@ public:
 private:
   std::string constraint_name;
   utils::constraints options_{utils::constraints::None};
+  std::shared_ptr<object> reference_;
   std::string column_name;
   std::string ref_table_name;
   std::string ref_column_name;