The wrong function was being assigned to the Hessian element 'data.calc_d2jw[0][data.s2f_li]' for
the optimisation of the spherical diffusion parameters together with the parameters S2f, tf, S2, and
ts. The function used was
'calc_diff_S2f_S2_ts_d2jw_dDjdS2f',
whereas it should have been,
'calc_diff_S2f_tf_S2_ts_d2jw_dDjdS2f'.