Err and correctly handle complex returns in reverse mode

[wsmoses]

@giordano @MasonProtter

resolves #1308

[MasonProtter]

Great stuff! One area that might be good to discuss, but we could also just skip is discussing that for general non-holomorphic functions, you can compute the pair of derivatives (d/dz, d/dz̄) instead of (d/dx, d/dy). For a holomorphic function you'll have df/dz̄ = 0 and for an anti-holomorphic function you'll get df/dz = 0

In general these things can be computed as

df/dz = (pullback(f, z)(1)' + pullback(f, z)(im)' * im)/2
df/dz̄ = (pullback(f, z)(1)' + pullback(f, z)(im)' * im)/2

People sometimes call this pair of derivatives 'Wirtinger' derivatives.

If you want I can try to write a quick paragraph on it for the docs.