Locate the plane for simpx of on wulff_cv, by comparing the center of
        the simpx triangle with the plane functions.

def _get_simpx_plane(self):
        """
        Locate the plane for simpx of on wulff_cv, by comparing the center of
        the simpx triangle with the plane functions.
        """
        on_wulff = [False] * len(self.miller_list)
        surface_area = [0.0] * len(self.miller_list)
        for simpx in self.wulff_cv_simp:
            pts = [self.wulff_pt_list[simpx[i]] for i in range(3)]
            center = np.sum(pts, 0) / 3.0
            # check whether the center of the simplices is on one plane
            for plane in self.facets:
                abs_diff = abs(np.dot(plane.normal, center) - plane.e_surf)
                if abs_diff < 1e-5:
                    on_wulff[plane.index] = True
                    surface_area[plane.index] += get_tri_area(pts)

                    plane.points.append(pts)
                    plane.outer_lines.append([simpx[0], simpx[1]])
                    plane.outer_lines.append([simpx[1], simpx[2]])
                    plane.outer_lines.append([simpx[0], simpx[2]])
                    # already find the plane, move to the next simplices
                    break
        for plane in self.facets:
            plane.outer_lines.sort()
            plane.outer_lines = [line for line in plane.outer_lines
                                 if plane.outer_lines.count(line) != 2]
        return on_wulff, surface_area