Deal with multi-use facilities

[fredshone]

Objects tend to be assigned with multiple activities due to the mappings supporting 1:n and building additionally having multiple relevant tags.

We currently output a geojson where the activities feature is a list of assigned activities.

We should additionally output or post process these 'shared activity' objects into unique objects. The snag is then that we may also want to support changing the object features, for example if a building is both a residence and a shop we might split the floor area between them. However other activity types such as delivery would not merit this.