Implement QC stage 1

QC stage 1 is for each admin1 or admin2 location, find the GAUL admin unit whose centroid is closest to the location (as long as it's within a certain distance).

Also:
- Fix centroids in admin_unit table - many of these were incorrectly rounded.
- Map GeoNames feature codes "ADMD" and "ADMH" to ADMIN2 rather than ADMIN1.
- DataAcquisition now always returns COUNTRY precision if there is no GeoNames ID and the place name is the name of a HealthMap country.