add GA6 conformance rule

In `GeographicalArea`
>Loops in the parent relation between geographical areas and parent geographical area groups are not allowed. If a geographical area A is a parent geographical area group of B then B cannot be a parent geographical area group of A (loops can also exist on more than two levels, e.g. level 3; If A is a parent of B and B is a parent of C then C cannot be a parent of A).