Get indices of neighbour simplices for each simplex and arc indices.
        Identical to get_neighbour_simplices() but also returns an array
        of indices that reside on boundary hull, -1 denotes no neighbour.

def neighbour_and_arc_simplices(self):
        """
        Get indices of neighbour simplices for each simplex and arc indices.
        Identical to get_neighbour_simplices() but also returns an array
        of indices that reside on boundary hull, -1 denotes no neighbour.
        """
        nt, ltri, lct, ierr = _tripack.trlist(self.lst, self.lptr, self.lend, nrow=9)
        if ierr != 0:
            raise ValueError('ierr={} in trlist\n{}'.format(ierr, _ier_codes[ierr]))
        ltri = ltri.T[:nt] - 1
        return ltri[:,3:6], ltri[:,6:]