Fixes for the lib.dispersion.ns_2site_expanded module.
These problems were identified using the Relax_disp.test_hansen_cpmg_data_ns_2site_expanded system
test. They correspond to the issues with the original fitting_main_kex.py program identified by
Mathilde Lescanne in her post at http://thread.gmane.org/gmane.science.nmr.relax.devel/4144.
