Convenience function to add an offset column to phi.
        You can use this function to add an offset (bias on y axis)
        to phi in your custom self._phi(X).

def concatenate_offset(self, X):
        """
        Convenience function to add an offset column to phi.
        You can use this function to add an offset (bias on y axis)
        to phi in your custom self._phi(X).
        """
        return np.c_[np.ones((X.shape[0], 1)), X]