Implement `TrialData.objectives` accessor

[motus]

trafficstars

We already have the ExperimentData.objectives, but it might be convenient to get that data on the Trial level, too. (Note that the objectives for a trial may be different from the ones of the experiment it belongs to)

[bpkroth]

I commented on this in #726, but I don't know as though we need this. ExperimentData.objectives seems more useful/relevant to me given a Trial can be reused across multiple Experiments even if they have different objectives.