Stop exporting metrics?

[ablaom]

Currently we export a bunch of metrics which are not part of core functionality and which have a reasonable probability of name conflicts with other packages used with DecisionTree.jl - if not now, then maybe in the future:

export majority_vote, ConfusionMatrix, confusion_matrix, mean_squared_error, R2

Of course removing the export is breaking. But we are poised to make a breaking release soon.

What do others think about this suggestion?