write a flopy.utils.Util2D instance to an ASCII text file using the
        Util2D filename

        Parameters
        ----------
        u2d : flopy.utils.Util2D

        Returns
        -------
        filename : str
            the name of the file written (without path)

def write_u2d(self, u2d):
        """ write a flopy.utils.Util2D instance to an ASCII text file using the
        Util2D filename

        Parameters
        ----------
        u2d : flopy.utils.Util2D

        Returns
        -------
        filename : str
            the name of the file written (without path)

        """
        filename = os.path.split(u2d.filename)[-1]
        np.savetxt(os.path.join(self.m.model_ws,self.arr_org,filename),
                   u2d.array,fmt="%15.6E")
        return filename