Adapt A-FVS to work with a single set of nodes

[netotz]

Currently AlphaFastVertexSubstitution only works with InstanceTwoSets, so it must be adapted to work with InstanceSameSet too to compare against the literature.

Ideas

Since $f_r$ would become a user $u_r$, it should be allocated to its $\alpha$-neighbors and update $x'$, $r(\cdot)$ and $z(\cdot)$ according to its $\alpha$-th center, because this new assignment could be a better objective function value.