add GA5 conformance rule

on `GeographicalArea`
>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.

It won't complain if dates aren't present
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).
enhance GA5 and GA6 specs

add 2 more tests to GA5
Merge pull request #168 from bitzesty/GA6

add GA5 and GA6 conformance rules