Function expected_improvement 

pub fn expected_improvement(
    gp: &GaussianProcess,
    x: &[f64],
    best_so_far: f64,
    exploration: f64,
) -> Result<f64, String>

Expected Improvement acquisition at a query point.

z = (μ − f_best − ξ) / σ; EI = (μ − f_best − ξ)Φ(z) + σφ(z). ξ > 0 biases exploration.