Return a copy of the array with chosen storage type.

        Returns
        -------
        NDArray or RowSparseNDArray
            A copy of the array with the chosen storage stype

def tostype(self, stype):
        """Return a copy of the array with chosen storage type.

        Returns
        -------
        NDArray or RowSparseNDArray
            A copy of the array with the chosen storage stype
        """
        # pylint: disable= no-member, protected-access
        if stype == 'csr':
            raise ValueError("cast_storage from row_sparse to csr is not supported")
        return op.cast_storage(self, stype=stype)