Function feature_wise_mean_std 

pub fn feature_wise_mean_std(
    dataset: &DataSet,
    columns: &[&str],
    std_kind: VarianceKind,
) -> Option<Vec<(String, FeatureMeanStd)>>

One pass over all rows: compute mean and std dev for each listed numeric column (Int64 / Float64). Nulls are ignored. If a column has no non-null values, both fields are Value::Null. Sample std dev is undefined for fewer than two values → Value::Null.

Returns None if any name is missing from the schema or is not numeric.