Function beer_lambert_multi 

pub fn beer_lambert_multi(
    cross_sections_per_isotope: &[&[f64]],
    thicknesses: &[f64],
) -> Result<Vec<f64>, TransmissionError>

Compute transmission for multiple isotopes.

T(E) = exp(-Σᵢ thicknessᵢ × σᵢ(E))

Arguments

• cross_sections_per_isotope — Vec of cross-section arrays, one per isotope. Each inner slice has the same length as the energy grid.
• thicknesses — Areal density (atoms/barn) for each isotope.

Returns

Combined transmission values at each energy point.