Function resolve_split 

pub(crate) fn resolve_split(
    k: Complex,
    lattice: &BlochLattice,
    eta: Complex,
) -> Complex

The Ewald split: eta, or for zero an automatic choice from kL with the cell length L = measure^(1/dim). The exact sum is independent of the split, so it is held fixed during differentiation instead of differentiating the heuristic.