Added gde_buildings table

Merged Cecilia Nievas requested to merge feature/add_gde_buildings into master

The gde_buildings table stores, for each OBM building, the building's classes and their associated probabilities.

Information on aggregated_source_id, occupancy_case and data_unit_id is necessary to be able to retrieve number of people per building and cost per building from the data_units_buildings table.

In the future the code could also be modified so that the columns building_class_names, settlement_types, occupancy_subtypes and probabilities are empty when the OBM building is assigned all building classes associated with the corresponding aggregated_source_id, occupancy_case and data_unit_id, which can then be directly retrieved from the data_units_buildings table.

Associated with gde-core!17 (merged).

\approve @fd

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