Add total_builtup_area to the boundaries table and add average_floorspace to the district_assets table

The total built-up area is a column in the boundaries table already implemented in PostGIS. We need to add it to SpatiaLite too. It is the total expected building footprint that is computed by summing up the built-up area in all tiles inside the district.

The average floorspace should be added to the district_assets table, because all values (population / replacement costs) in the district_assets are now per building. If we know the average area, we can on-the-fly compute the values for a building that is bigger or smaller.

\fyi @all