Added the IT99 model equations to the relax library.
This is the Ishima and Torchia 1999 2-site model for all timescales with pA >> pB.
This commit follows step 3 of the relaxation dispersion model addition tutorial (http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
Authored by: bugman 2013-06-10
Parent: [r20007]
Child: [r20009]
