This is the model of the numerical solution for the 2-site Bloch-McConnell equations. It originates
as optimization function number 1 from the fitting_main_kex.py script from Mathilde Lescanne, Paul
Schanda, and Dominique Marion (see http://thread.gmane.org/gmane.science.nmr.relax.devel/4138,
https://gna.org/task/?7712#comment2 and https://gna.org/support/download.php?file_id=18262).
This commit follows step 4 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
