Make `MAPElitesRepertoire` more modular so that it can store any kind of information.

[Lookatator]

Make MAPElitesRepertoire more modular so that it can store any kind of information (e.g. observations for AURORA).

The idea would be to add some extra_infos attribute to it (type: Dict[str, pytree]).

Also the repertoire should take as input a tuple of the fields (type: Tuple[str]), and only save the items of the batch extra_infos whose keys are equal to the fields passed as input. This way we can avoid storing too much in the repertoires.