alphagov/trade-tariff-backend — pull request #168 by macool (2014-04-24 19:11:27+00:00)
Title: add GA5 and GA6 conformance rules

To `GeographicalArea`
**GA5**
>If a geographical area has a parent geographical area group then the validity period of the parent geographical area group must span the validity period of the geographical area.

Note: It won't complain if dates aren't present

**GA6**
>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)