Added feature to write data units attributes to database table

gdeimporter/aggregatedexposuremodel.py

gdeimporter/dataunit.py

gdeimporter/gdeimporter.py

tests/conftest.py

...s/data/config_for_testing_exposure_entities_code_iso3.yml

tests/data/config_for_testing_good.yml

tests/data/test_database_aggregated_sources.sql

tests/data/test_database_built_up.sql

tests/data/test_database_set_up.sql

tests/test_aggregatedexposuremodel.py

tests/test_configuration.py

tests/test_dataunit.py