Created a special list-like structure for spin specific data.
This is in response to the post of Pavel Kaderavek <pavel dot kaderavek att gmail dott com>
at https://mail.gna.org/public/relax-devel/2011-02/msg00071.html (Message-id:
<AANLkTi=DQ9KGP+e0ONhZ66XhSzKhuQzFoQcQNwXnVsgG@...>).
