Import reference exposure from PostGIS database

Currently, the function import_from_postgis only imports the tables Asset, Entity and Taxonomy from the PostGIS server database to a spatialite local database.
In order to import the baseline or reference exposure model (the exposure model that is created after running the initializer; which would be the aggregated exposure model distributed over the tiles and includes no building information) we need to have a function that imports from the AssetReference, EntityReference and Taxonomy tables.

\fyi: @ds @shinde @laurens