Function quantile_indices 

pub fn quantile_indices(
    n_nonnull: usize,
    quantile: f64,
    method: QuantileMethod,
) -> (usize, usize, f64)

Returns the 0-based indices into n_nonnull sorted non-null values that quantile needs, and the fraction of the way from the lower to the upper one: (lower_idx, upper_idx, fraction).

upper_idx == lower_idx unless method combines two distinct values. Only Linear uses fraction; Midpoint averages the two values.