Math-domain catching for model: 'NS CPMG 2-site expanded'.
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 can be found via the --numpy-raise function to the systemtests.
To make the code look clean, the class object "back_calc" is no longer
being updated per time point, but is updated in the relax_disp target function in
one go.
