For more info, see my post at https://mail.gna.org/public/relax-devel/2006-10/msg00057.html
(Message-id: <1160557041.9523.74.camel@...>). The idea of the
generator function, using the yield statement, to return the spin data containers was given by Gary
Thompson at https://mail.gna.org/public/relax-devel/2007-01/msg00014.html (Message-id:
<f001463a0701071417w6bd7927cp8fdd052e698575ec@...>). More details are given by Chris
MacRaild at https://mail.gna.org/public/relax-devel/2007-01/msg00036.html (Message-ID:
<1168883717.7569.511.camel@...>).
The full generator function is generic_fns.selection.spin_loop().
