construct the model matrix columns for the term

        Parameters
        ----------
        X : array-like
            Input dataset with n rows

        verbose : bool
            whether to show warnings

        Returns
        -------
        scipy sparse array with n rows

def build_columns(self, X, verbose=False):
        """construct the model matrix columns for the term

        Parameters
        ----------
        X : array-like
            Input dataset with n rows

        verbose : bool
            whether to show warnings

        Returns
        -------
        scipy sparse array with n rows
        """
        columns = super(FactorTerm, self).build_columns(X, verbose=verbose)
        if self.coding == 'dummy':
            columns = columns[:, 1:]

        return columns