Postgres Support

Merge request to support Postgres #1285

• update db_common.py
• update db_manager.py
• update db_structure.py
• Add pytest tests
• update database_manager.py
• update experiment_status_db_manager.py
• update experiment_history_db_manager.py
• check nullable values that are not nullable (sqlite does not enforce & AS code is writing nulls!)
• perform tests listed in the issue #1285