This is the numerical model for the 2-site Bloch-McConnell equations expanded using Maple by Nikolai
Skrynnikov. It originates as optimization function number 5 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 3 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
