Write a merge request to osm2geojson dependency to support 'center' coordinates

Action: Add tests and write a merge request to osm2geojson dependency to support 'center' coordinates.

Background:

• As described in #8 (closed) in this plugin there's a typical use case, where OSM objects and POIs (e.g. a hospital or a castle) are of same type but have been mapped with area geometry.
• The current solution is that the Overpass query contains "out center;" so that the response contains "center" coordinates in addition to the ways or relation.
• Unfortunately the Python lib osm2geojson doesn't support this center (opposite to the Javascript lib), so we had to patch it: https://gitlab.com/geometalab/transfer-praktikanten-2019/OSMDataSync/-/blob/master/OSMDataSync/dependencies/osm2geojson/main.py#L174 ff.