Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self,
                   'images_processed') and self.images_processed is not None:
            _dict['images_processed'] = self.images_processed
        if hasattr(self, 'images') and self.images is not None:
            _dict['images'] = [x._to_dict() for x in self.images]
        if hasattr(self, 'warnings') and self.warnings is not None:
            _dict['warnings'] = [x._to_dict() for x in self.warnings]
        return _dict