Hmm... Sticking all the scores in one field has its advantages, but it's not clear how we fit training scores or times in here without changing the length of the namedtuple (breaking forwards compatibility), or without somehow modifying and restructuring the namedtuple returned by the scorer. I still think `_CVScoreTuple` has to go. But that may not be within the scope of this PR (but is one reason I think the multiple metrics thing isn't within the scope of this PR, either).