Function eccentricity_from_apsides 

pub fn eccentricity_from_apsides(
    periapsis_radius: f64,
    apoapsis_radius: f64,
) -> Option<f64>

Computes eccentricity from periapsis and apoapsis radii.

Formula: e = (r_a - r_p) / (r_a + r_p)

Returns None when either radius is less than or equal to zero, when apoapsis_radius is less than periapsis_radius, or when the input or result is not finite.