Function delta_energy 

fn delta_energy(
    h: &[(usize, f64)],
    j_by_qubit: &[Vec<(usize, f64)>],
    spins: &[i8],
    qubit: usize,
) -> f64

Delta energy ΔE = E_after - E_before for flipping spin qubit from s_q to -s_q in the Ising model H = Σ h_i s_i + Σ J_ij s_i s_j.

ΔE = −2·s_q·(h_q + Σ_k J_qk·s_k).