Convert the object into a json serializable dictionary.

        :param boolean save_data: if true, it adds the training data self.X and self.Y to the dictionary
        :return dict: json serializable dictionary containing the needed information to instantiate the object

def to_dict(self, save_data=True):
        """
        Convert the object into a json serializable dictionary.

        :param boolean save_data: if true, it adds the training data self.X and self.Y to the dictionary
        :return dict: json serializable dictionary containing the needed information to instantiate the object
        """
        input_dict = super(SparseGP, self).to_dict(save_data)
        input_dict["class"] = "GPy.core.SparseGP"
        input_dict["Z"] = self.Z.tolist()
        return input_dict