Added the 8th unit tests demonstrating edge case 'no Rex' failures of the model 'DPL94'.
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.
This is to implement catching of math domain errors, before they occur.
These tests cover all parameter value combinations which result in no exchange:
