Optimizer-related extra-data

[mb706]

Archive should have a field for data that a specific optimizer might need but that can not easily be written into more columns of the $data. An example would be multi-objective GAs, where it is useful to save the dominated hypervolume of a generation (whole batch of evaluations). This hypervolume does not belong to any specific individual and therefore does not make sense to have in the $data. I would need this for mlr-org/miesmuschel#86.