Function unbroadened_cross_sections 

pub fn unbroadened_cross_sections(
    energies: &[f64],
    resonance_data: &[ResonanceData],
    cancel: Option<&AtomicBool>,
) -> Result<Vec<Vec<f64>>, TransmissionError>

Compute unbroadened (raw Reich-Moore) cross-sections for each isotope.

This is the temperature-independent first step of the forward model. The result can be cached and reused across multiple temperature evaluations (e.g., during LM iterations where temperature is a free parameter).

Returns

One total cross-section vector per isotope: result[k][e] is the unbroadened total cross-section (barns) for isotope k at energy e.