Resolve "Problems with wrong occupancy"

Closes #34 (closed)

Some higher-priority datasets, like 3D datasets require to use the osm_tags in the OccupancyRule, otherwise the intersected OSM buildings and their tags will be rejected in the conflation. The consequence is when no occupancy from 3D datasets and OSM information is excluded, the building geometry takes the occupancy from the lower-priority sources like EUBucco or GHSL.

Also, rename the original osm_spots to osm_poi, and osm_lands to osm_landuse, based on the naming in refactored rule OSMContextOSMSource and OSMContextOtherSource.

Since the new rule for non-OSM datasets named as OccupancyFromOther, we rename the original occupancy rule to OccupancyFromOSMRule.

\approve @laurens @ds