Fixed some bugs; added example of creating/removing a DN link

Extended the DN type system to retrieve link types; these are now included in the oslcquery --typesystemreport
Fixed issues in reqif_io: selecting a config didn't work, and there was a typo for QUERY_PASSWORD
Added example db_simple_modifylink.py - this adds/removes a DN link between core artifacts; this is what required extending the typesystem to retrieve linktypes, so the link type to be created could be found by name