remove BuildingGroup#granularity

The granularity attribute (representing whether a building group refers to a specific postcode or the whole city) is a bit misleading and, as discussed with otfrom, can be inferred from the presence of a value in the postcode attribute.
add BuildingGroup#location_scope

#location_scope will return either "City wide" or "Postcode: xxx" based on the presence of the postcode attribute in the BuildingGroup.