Georef: fix angle computation

we did not check if a == c in the angle computation thus giving wrong
directions.

fix http://jira.canaltp.fr/browse/NAVITIAII-1044
Merge pull request #555 from antoine-de/fix_angle

Georef: fix angle computation
jormun: error code in doc was not up to date
Merge pull request #557 from antoine-de/update_doc_error_code

jormun: error code in doc was not up to date
georef: remove useless rtree
georef: add find_admins and use it
georef: move GeoRef::build_admins_{stop_points,pois} into Data::build_administrative_regions
cities: add sane default for alembic.ini
ed: add support of the cities database in ed2nav to add admins to sp, sa and poi