Override standard get_output to return xml-file if xml-file is specified.
    Otherwise, will return database.

def get_output(self):
    """
    Override standard get_output to return xml-file if xml-file is specified.
    Otherwise, will return database.
    """
    if self.__xml_output:
      return self.__xml_output
    elif self.get_database():
      return self.get_database()
    else:
      raise ValueError, "no output xml file or database specified"