Adding code to allow reordering the NN atom list.
Adding code to allow reordering the NN atom list.

If the `reference_atom` attribute is set, then the code
will attempt to return the `NN` attribute list with the same
atom order as the `reference_atom.NN` atom list.

scikit-nano/scikit-nano,androomerrill/scikit-nano,scikit-nano/scikit-nano,androomerrill/scikit-nano