`MAPElitesRepertoire` sampling function should be more modular.

[Lookatator]

MAPElitesRepertoire sampling function should be more modular.

For instance, the sampling function can return a pytree of type MAPElitesRepertoire, with all the corresponding information (genotypes, fitnesses, descriptors, and so on...)

This way, no need to implement new functions (as currently done for some algorithms) every time we need to sample genotypes with extra information.