Add 'Demographic Group' (NCIT:C161324) as new top-level class

Add 'Demographic Group' (NCIT:C161324) as a new top-level class in place of 'Ethnic Group' (NCIT:C16564) and 'Race'(NCIT:C17049).

Note: 'Ethnic Group' and 'Race' are subclasses of 'Demographic Group'.