Introduce subpopulation schemes

[jakobbossek]

To maintain diversity and prevent premature convergence one can maintain multiple subpopulations, which is known as island model or regional model in the literature. Here different subpopulations evolve independently and occasionally exchange individuals. This way one might prevent the algorithm to converge to early in a local minimum.

We should think about the introduction of simple regional schemes. At least the following things should be introduced:

• the number of (sub)populations
• isolation time, i. e., the number of generations in which the subpopulations evolve independently without exchanging individuals
• migration rate, i. e., the number or the fraction of individuals shared in the migration phase
• migration selection mechanism, i. e., which elements (best/random/...) should be interchanged between the subpopulations