Reverted a change so that the 'relax_sim_data' structure is correctly called.
While making changes for the new design, it had been accidentally changed to 'relax_sim_spin'.
This was pointed by Edward d'Auvergne in a post at:
https://mail.gna.org/public/relax-devel/2008-01/msg00051.html (Message-id:
<7f080ed10801140725l5422b067jcd6c6d050fbb6e77@...>)
