Resolve differences between PostGISExposure implementation and SpatiaLiteExposure

In spatialite.py the function create_tables() is responsible for creating the necessary tables for exposure data. It still contains the obsolete baseline_assets table, which should be removed.

Further differences between PostGIS and SpatiaLite:

• Table entities doesn't have a geometry3d column

• Table entities is missing indexes

• Table district_assets is referencing table taxonomy (should be taxonomies) \fyi @laurens @gislars @pomodoro @pdlmora