Changed the nucleus type and gyromagnetic ratio handling to the new design.
This is similar as in "specific_fns/model_free/mf_minimise.py" (line 1154, r5603) and as explained
in a post by Edward d'Auvergne at:
https://mail.gna.org/public/relax-devel/2008-04/msg00055.html (# Message-id:
<1208028698.8064.23.camel@...>)
