Function spectral_function 

pub fn spectral_function<H, P>(
    target_hamiltonian: &H,
    source: &[Complex64],
    probe: &P,
    options: SpectrumOptions,
) -> Result<Vec<f64>>
where
    H: LinearOperator + ?Sized,
    P: LinearOperator + ?Sized,

Lorentzian-broadened spectral density in a same or different target sector.