Add relation to parent/child countries now that we have this information in subdivisions

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (4592592) to head (6e4ed5f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #239 +/- ## =========================================== Coverage 100.00% 100.00% - Complexity 274 287 +13 =========================================== Files 43 43 Lines 9641 9664 +23 =========================================== + Hits 9641 9664 +23 ```

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