pub fn AUC(dv: &DataVec, predict: &PredVec, len: usize) -> ValueType
AUC (Area Under the Curve) calculation for first n element in data vector. See wikipedia for detailed algorithm.
If the specified length is greater than the length of data vector, it will panic.
If the length of data vector and predicted vector is not same, it will panic.
If the data vector contains only one class or more than two classes, it will panic.