Added the 'M61 skew' model to the relax_disp.select_model user function frontend.
This is the Meiboom 1961 model for skewed populations (pA >> pB).
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 2013-06-08
Parent: [r19968]
Child: [r19970]
