use dispatch-based selectors for customizable parts of SGM

[brunompacheco]

No description provided.

[brunompacheco]

I just realized this would make the customization more convoluted for the user that wants to use custom methods inside the SGM algorithm. For example, in the current approach, the user could simply do

function get_player_random_order(players::Vector{Player}, iter::Integer, Σ_S::Vector{Profile{DiscreteMixedStrategy}}, payoff_improvements::Vector{Tuple{Player,Float64}})
    return shuffle(keys(players))
end
IPG.get_player_order = get_player_random_order

This would become significantly more complicated if the user had to handle dispatch.