Added the 'TAP03' R2eff calculating function to the relax library.
This is the Trott, Abergel and Palmer 2003 R1rho analytic model for 2-site off-resonance exchange.
This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_relax_library.
