Critical fix, for k_AB not belonging to list of global parameters.
k_AB was only changed to the spin of interest, but not for the rest of the cluster.
When the parameter vector is assembled, "assemble_param_vector(spins=spins)" it takes the global parameter from spin 0.
Bug #22754 (https://gna.org/bugs/index.php?22754): The minimise.calculate() does not calculate chi2 value for clustered residues.
