Sets unique id for this epub

        :Args:
          - uid: Value of unique identifier for this book

def set_identifier(self, uid):
        """
        Sets unique id for this epub

        :Args:
          - uid: Value of unique identifier for this book
        """

        self.uid = uid

        self.set_unique_metadata('DC', 'identifier', self.uid, {'id': self.IDENTIFIER_ID})