Created the Relax_disp.test_ns_mmq_3site_linear system test and modified Relax_disp.test_ns_mmq_3site.

The Relax_disp.test_ns_mmq_3site_linear system test uses the old data from the directory
test_suite/shared_data/dispersion/ns_mmq_3site/, as this had kAC = 0, now copied into the
ns_mmq_3site_linear/ directory.  This system test uses the 'NS MMQ 3-site linear' model.

The base data generated by cpmg_fit for the Relax_disp.test_ns_mmq_3site system test was modified so
that kAC is no longer 0, but set to 1000.  This should properly test the 'NS MMQ 3-site' model.
Renamed the 'MMQ 2-site' model to 'NS MMQ 2-site'.

This is so that the name matches those of the 'NS MMQ 3-site linear' and 'NS MMQ 3-site' models.
Renamed all remaining instances of 'MMQ 2-site' to 'NS MMQ 2-site'.

This is simply changing variable, method and module names.
Removed the 'MMQ 3-site branched' and 'MMQ 3-site linear' models from the to do list in the manual.

These two dispersion models are now implemented.
Renamed the 'MQ CR72' dispersion model to 'MMQ CR72'.

The model is designed by Korzhnev et al., 2004 for proton-heteronuclear SQ, ZQ, DQ, and MQ data
(or MMQ data), so the change is logical as the model is not just for MQ data.
Clean up of the 'NS R1rho 3-site' model names in the manual.

The word 'branched' has been removed and the notation now matches the 'NS MMQ 3-site' models.
Clean up of the parameter lists in the dispersion model table of the manual.
The pC parameter constraints are now implemented for the 3-site dispersion models.

The new constraints are 0 <= pC <= pB.