Function variance_per_atom_material 

pub fn variance_per_atom_material(
    model: StragglingModel,
    charge: EffectiveCharge,
    ion: &Ion,
    material: &Material,
    energy_ev: f64,
) -> Result<f64, StoppingError>

Bragg-additive variance per average atom of a material, J² m² (Σ_j x_j Ω_j²).