Added the DPL94 model equations to the relax library.
This is the David, Perlman and London 1994 R1rho 2-site fast exchange model.
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 2013-06-09
Parent: [r19999]
Child: [r20001]
