Allow parents with high fitness to have more offspring per generation

[Boscop]

Currently, the crossover method can only return one offspring.
But often it is desirable to have high fitness parents producing more offspring.
And it's not just enough to call the crossover function multiple times, because often the offspring should be generated exhaustively, regarding a certain set of "orthogonal" crossover combinations that makes sense, without repeating the same way of crossover.
Simply calling crossover multiple times would potentially produce the same offspring multiple times, if the number of crossover-ways that make sense is low.
The same can be said for the mutate method. Often it's desirable to generate multiple "orthogonal" mutations together.