Fix for systemtest Relax_disp.test_exp_fit(), where the spin.isotope was not set.
The new call to return_r2eff_arrays(), when producing graphs, raise RelaxSpinTypeError() if no isotope is set.
bug #22461(https://gna.org/bugs/?22461): NS R1rho 2-site_fit_r1 has extremely high chi2 value in systemtest Relax_disp.test_r1rho_kjaergaard_missing_r1.
