Function oberth_efficiency 

pub fn oberth_efficiency(
    mu: Mu,
    r_periapsis: Km,
    v_inf: KmPerSec,
    burn_dv: KmPerSec,
) -> f64

Fractional Oberth efficiency: (Δv_inf / burn_dv) - 1.

Returns the fractional improvement in v_inf change relative to the burn magnitude. For example, 0.03 means 3% more effective than a burn in free space.