Get POI "area" from enclosing building

[zerebubuth]

Some POIs currently don't have an area as they are generated from points. However, we have building data and can join onto that to recover area. POIs should be collected by the smallest building polygon they intersect (but not building parts?) and the area of the polygon shared between them, respecting existing areas.

This might run into some trouble with multi-tenanted buildings (e.g: one POI per-floor), but is probably a good place to start iterating.

[nvkelso]

This doesn't affect the API, moving to v1.1