Add multithreading support in RF predictions: probabilistic classification - and regression

[ablaom]

When multithreading prediction support was added in https://github.com/JuliaAI/DecisionTree.jl/pull/188/files it appears that it was only added for apply_forest and not apply_forest_proba. And not added at all for regression, only classification.

[ablaom]

@salbert83 Any chance you would have time and inclination to take this on?

[salbert83]

I should have some time on 2023-01-15 to look. Thanks