Function spearman_corr

crates/openquant/src/feature_importance.rs:447–451

(x: &[f64], y: &[f64])

    let mut out = BTreeMap::new();
    for (name, column) in feature_names.iter().zip(proba) {
        let model = Column(column);
        let mut scores = Vec::with_capacity(splits.len());
        for (_, test) in splits {
            // Indices up to 2^53 are exact as f64.
            let x_test: Vec<Vec<f64>> = test.iter().map(|&i| vec![i as f64]).collect();
            let y_test = vals(y, test);
            let sw_test = sample_weight.map(|sw| vals(sw, test));