Methods to replace math domain errors in model ns_cpmg_2site_expanded, has been replaced with numpy masks.
number of points has been removed, as the masks utility replaces this.
pB is now moved to be calculated inside. This makes the lib function simpler.
k_AB and k_BA is also now calculated here.
Documentation is also fixed.
Task #7807 (https://gna.org/task/index.php?7807): Speed-up of dispersion models for Clustered analysis.
