use admin_centre node for relation centroid where available

[missinglink]

as per discussion in #96 some relations have a node member of role 'admin_centre' which we should favour over the computed geographic centroid.

[timonmasberg]

Wouldn't it be better if we check that the relation is of type "boundary":"administrative" (that's where admin_centre is of relevance) before we go through every member for every relation?

[missinglink]

Wouldn't it be better if we check that the relation is of type "boundary":"administrative" (that's where admin_centre is of relevance) before we go through every member for every relation?

It's really not very expensive since the members are already in memory and there's only one per immediate member (so one per ring), it probably only takes a fraction of a millisecond to compute.

That being said, if it's only relevant for boundary=administrative then we can only apply it on those, are we 100% sure that's correct? I had a look at the wiki and it seems logical.

[missinglink]

Added a boundary=administrative check via rebase.

[missinglink]

go run ./... -tags 'official_status:de~Land' /data/osm/hamburg-latest.osm.pbf | jq .centroid

{
  "lat": "53.5437641",
  "lon": "10.0099133",
  "type": "admin_centre"
}