Returns index of nearest neighbor atoms.

def _find_nn_pos_before_site(self, siteindex):
        """
        Returns index of nearest neighbor atoms.
        """
        alldist = [(self._mol.get_distance(siteindex, i), i)
                   for i in range(siteindex)]
        alldist = sorted(alldist, key=lambda x: x[0])
        return [d[1] for d in alldist]