How do we want to handle multiple labels?

[MatthewJA]

How do we want to handle multiple labels? The predictor input currently supports multiple labels, but I'm not sure what we should do to collate these. Ideally we can swap something out e.g. majority vote, but would this be on the Predictor? Part of the PredictorInput? Or its own thing entirely?

I can also just force the number of labellers to be 1 (like I've done for multidimensional labels, which are supported but explicitly disallowed until we have a way to deal with them).