Included obm_built_area_size column in obm_tiles

With this command the obm_built_area_size column is added to obm_tiles. It contains built up areas based on the portion of the OBM building footprints that intersect with a tile. also projected to Albers Equal Area.

\approve @ds @fd

migrations/00009_add-obm-built-area-size-column.sql

+-- Create the `obm_built_area_size` column in obm_tiles
+
+ALTER TABLE public.obm_tiles
+    ADD built_area_size_obm FLOAT;