Added functionality to erase existing entries in gde_buildings table

So far, the gde-core did erase first all entries that may have already existed in the table gde_buildings of the GDE Tiles database for a particular combination of data_unit_id, occupacy_case and aggregated_source_id, before moving on to processing and storing new data. However, the actual data unit IDs associated with an exposure entity may change for different versions of the aggregated exposure model, and it may happen that the user wants to treat different versions under the same aggregated_source_id. This MR addresses the need to first erase all entries associated with a specific exposure_entity, occupancy_case and aggregated_source_id, before moving on to writing new entries.