Return a newly created |NotesSlidePart| object related to this slide
        part. Caller is responsible for ensuring this slide doesn't already
        have a notes slide part.

def _add_notes_slide_part(self):
        """
        Return a newly created |NotesSlidePart| object related to this slide
        part. Caller is responsible for ensuring this slide doesn't already
        have a notes slide part.
        """
        notes_slide_part = NotesSlidePart.new(self.package, self)
        self.relate_to(notes_slide_part, RT.NOTES_SLIDE)
        return notes_slide_part