Update the optimality indicators

[dongzhiming]

The optimality indicator $f_i$ of the instance $x_i$ is updated to $f^{'}_ i$ using the following formula:
$f_i^{'}=f_{i} + (\alpha_{u}^{'} - \alpha_{u}) y_{u} K(x_u,x_i) + (\alpha_l^{'} - \alpha_l ) y_{l} K(x_l,x_i)$,
as shown in formula (13) in thundersvm-full.pdf, but at line 110 of the code smo_kernel.cpp, why the contribution of the two extreme instances $x_u$ and $x_l$ to $f_i$ is in the form of difference instead of summation, and in the update_f function, the update of $f_i$ is consistent with formula (13).