Return Maps from Cross Validation

[akleeman]

Cross validation now returns a map from the fold name to a predictions, instead of a vector of predictions. This makes it so we don't need to rely on preserved order.

[seth-swiftnav]

lgtm!
I've a small preference for [x] over .at(x) when referencing maps. But as mentioned, that might not be allowed for constant maps.

[akleeman]

yeah, operator [] doesn't work for const maps: https://stackoverflow.com/questions/1474894/why-isnt-the-operator-const-for-stl-maps