get an 2D instance of self with all zeros

        Returns
        -------
        Matrix : Matrix

def zero2d(self):
        """ get an 2D instance of self with all zeros

        Returns
        -------
        Matrix : Matrix

        """
        return type(self)(x=np.atleast_2d(np.zeros((self.shape[0],self.shape[1]))),
                   row_names=self.row_names,
                   col_names=self.col_names,
                   isdiagonal=False)