Can't replicate when using neutrinos

@dc-elbe1 do you see a reason why we impose this condition: https://gitlab.cosma.dur.ac.uk/swift/swiftsim/-/blob/master/src/space.c#L1130 and hence prevent box replication when nu's are used?

Replication is really about benchmarking so we don't especially care if the info in the nus is not the best.