allow creating many to one rules

For example:

```
gc.rule(['person.children.id', 'person.children.age'], 'person.oldest_child.id', cardinality='many-to-one')
def person_oldest_child(ids, ages):
    return argmax(ages, ids)
```

In this case, would a query which also fetches children and constrains them affect the oldest_child?
