Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'step') and self.step is not None:
            _dict['step'] = self.step
        if hasattr(self, 'snapshot') and self.snapshot is not None:
            _dict['snapshot'] = self.snapshot
        return _dict