Function analyze_crosstalk_bank 

pub fn analyze_crosstalk_bank(
    num_waveguides: usize,
    gap_nm: f64,
    coupling_length_um: f64,
    wavelength_nm: f64,
    core_index: f64,
    cladding_index: f64,
) -> CrosstalkBankResult

Analyse crosstalk in a uniform parallel-waveguide bank. Adjacent pairs (gap = g) are the dominant term; next-nearest (gap = 2g) are included as the largest secondary term — Marcatili 1969 predicts that all other pairs are at least exp(-2·g/L_decay) smaller still.