Added the 'MQ CR72' R2eff calculating function to the relax library.

This is the Carver and Richards (1972) 2-site model expanded for MQ CPMG data by Korzhnev et al.,
2004.

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.

The corresponding target function was updated to input the correct arguments.
Added the 'MQ CR72' model to the dispersion auto-analysis.

This is the Carver and Richards (1972) 2-site model expanded for MQ CPMG data by Korzhnev et al.,
2004.

This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_auto-analysis.
Added the 'MQ CR72' model to the GUI model list.

This is the Carver and Richards (1972) 2-site model expanded for MQ CPMG data by Korzhnev et al.,
2004.

This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_GUI.