Added feature to read and interpret exposure entities to run

The exposure entities to run can be indicated in different ways (via the exposure_entities_to_run parameter in the config file):

• asking gde-core to retrieve all exposure entities associated with the aggregated_source_id of the aggregated model selected to run
• writing the names of the exposure entities in the config file
• writing the names of the exposure entities in a CSV file whose name is indicated in the config file
• writing the names of the exposure entities in a TXT file whose name is indicated in the config file

The "conversion" of names to codes is handled by the exposure_entities_code parameter in the config file.