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.