Fix for looking for negative values, when all values where converted to positive in matrix in ns_cpmg_2site_3d.py.
This is to implement catching of math domain errors, before they occur.
This follows from the ideas in the post http://article.gmane.org/gmane.science.nmr.relax.devel/5858.
This is related to: task #7793: (https://gna.org/task/?7793) Speed-up of dispersion models.
