Created the DPL94 model target function.
This is the David, Perlman and London 1994 R1rho 2-site fast exchange model.
This commit follows step 4 of the relaxation dispersion model addition tutorial (http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
Authored by: bugman 2013-06-09
Parent: [r20000]
Child: [r20002]
