Compute entrances

[steinbro]

The original Soundscape tile server didn't just serve a subset of OSM objects; it also computed intersections and building entrances. We're currently computing intersections but not entrances.

This should just be a matter of translating the subquery at https://github.com/microsoft/soundscape/blob/main/svcs/data/tilefunc.sql#L28 from SQL into Python, as was done for intersections (see https://github.com/openscape-community/overscape-server/blob/main/app/overpass.py#L137).