ME: added a duration tag in common_tags.py
ME: added duration property in 'to_dict_for_mongo'
ME: added a test to check for '_duration' property in 'to_dict_for_mongo' in parsed_instance
ME: added missing '_duration' property
ME: changed return value to ''
ME: added missing '_duration' property and its value
ME: added missing '_duration' property and its value
ME: changed '_duration' value to ''
ME: updated fixtures
ME: added 'DURATION' tag to header list
ME: updated test with '_duration' key and its value