add Structure::what() -> "Structure" (for use in templates)

include/gemmi/model.hpp

@@ -925,6 +925,7 @@ inline const Entity* find_entity_of_subchain(const std::string& subchain_id,
 }
 
 struct Structure {
+  static const char* what() { return "Structure"; }
   std::string name;
   UnitCell cell;
   std::string spacegroup_hm;  // usually pdb symbol, cf. SpaceGroup::pdb_name()