Function lattice_expansion 

pub fn lattice_expansion(
    destination: Basis,
    source: Basis,
    ks: [Complex; 2],
    lattice: BlochLattice,
    eta: Complex,
) -> Result<(DMatrix<Complex>, LatticeExpansionResidual)>

Periodic cylindrical coupling with exact axial/polarization selection and cached orders.

Upstream: treams.cw.translate_periodic and the treams.ExpandLattice operator (treams.operators.expandlattice) between cylindrical bases.