Added functions to select the experiment type and mathematical model used.
These functions allow the user to select the experiment type (cpmg or r1rho) as well as the
mathematical model to fit the data (fast or slow).
These changes follow a discussion on relaxation dispersion, and mainly the post at:
https://mail.gna.org/public/relax-devel/2009-01/msg00020.html
(Message-id: <7f080ed10901070529s397f7f23p914c820ced4d07f@...>)
