Added the 'LM63 3-site' model to the relax_disp.select_model user function frontend.
This is the Luz and Meiboom 1963 analytic model for three exchanging sites.
This commit follows step 2 of the relaxation dispersion model addition tutorial (http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
Authored by: bugman 2013-07-19
Parent: [r20407]
Child: [r20409]
