addChild<T extends DataClass> method
This method registers a DataSet that will have direct relation to the dataset already instantiated. For that, it uses a relation name as a key for retrieving it later
Implementation
DataSet<Model> addChild<T extends DataClass>(String relationName, T instance, String routeTemplate) {
var child = DataSet<T>(instance, route: routeTemplate);
if (_children[relationName] != null) throw Exception();
_children[relationName] = child;
return this;
}