Implement predict_proba for DecisionTreeClassifier (#287) #539

Job	Run time
check_features (ubuntu, --features serde)	27s
tests (windows, x86_64-pc-windows-msvc)	5m 36s
check_features (ubuntu, --features datasets)	25s
tests (windows, i686-pc-windows-msvc)	3m 48s
check_features (ubuntu)	2m 36s
tests (ubuntu, x86_64-unknown-linux-gnu)	4m 10s
tests (ubuntu, i686-unknown-linux-gnu)	3m 3s
tests (ubuntu, wasm32-unknown-unknown)	4m 40s
tests (macos, aarch64-apple-darwin)	3m 58s
tests (ubuntu, wasm32-wasi)	3m 36s
	32m 19s

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