Function kambe 

pub(crate) fn kambe(n: i32, z: Complex, eta: Complex) -> Complex

The Kambe integral I_n(z, eta) without input checks.

The Kambe (1968) recurrence, as in treams, runs upward from the base pair I_-3, I_-1 (odd) or I_-2, I_0 (even); orders below it come from the series or the downward recurrence, whichever amplifies rounding less. Orders above -2 diverge at z = 0, all at eta = 0.