Added the 'MQ NS CPMG 2-site' R1rho calculating function to the relax library.

This is the 2-site numeric solution for multi-quantum CPMG-type data.

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.
Updated the 'MQ NS CPMG 2-site' model target function to match the function in the relax library.
Decreased the grid increments in the Relax_disp.test_sprangers_cpmg_data_auto_analysis system test.

This is to speed up this test.
Some small changes for the script for optimising Sprangers' ClpP MQ CPMG data.