Get subset of current Dataset.

        Parameters
        ----------
        used_indices : list of int
            Indices used to create the subset.
        params : dict or None, optional (default=None)
            These parameters will be passed to Dataset constructor.

        Returns
        -------
        subset : Dataset
            Subset of the current Dataset.

def subset(self, used_indices, params=None):
        """Get subset of current Dataset.

        Parameters
        ----------
        used_indices : list of int
            Indices used to create the subset.
        params : dict or None, optional (default=None)
            These parameters will be passed to Dataset constructor.

        Returns
        -------
        subset : Dataset
            Subset of the current Dataset.
        """
        if params is None:
            params = self.params
        ret = Dataset(None, reference=self, feature_name=self.feature_name,
                      categorical_feature=self.categorical_feature, params=params,
                      free_raw_data=self.free_raw_data)
        ret._predictor = self._predictor
        ret.pandas_categorical = self.pandas_categorical
        ret.used_indices = used_indices
        return ret