Support maximizing objective in `workon`, `report_objective`, etc.

[lebrice]

Instead of asking users to do something, we should do it ourselves. This is the kind of super basic thing that should absolutely be supported natively by Orion, without having to resort to a cheap *-1 trick that makes all commands report non-sensical negative accuracy values, etc.

[bouthilx]

Yeah our position was that we were making an MVP and max/min was quite easy for the users to handle so no need to include in Oríon. I think Oríon is clearly passed the MVP stage now, so it's about time. :sweat_smile: It would be great to handle this the same way we do with spaces, so that we invert objectives to be all minimized when passed to the algorithms. That would make algorithm implementation simpler.