caNonlinearPart = getNonlinearPart(caNonlinearPart, vEnvelopes, fPropagationCoordinateZ)
  dA(omega, kx, ky)/dz = L(omega, kx, ky)A(omega, kx, ky) + 
                       + F{ N(t, x, y, z) A(t, x, y) }
  this fnction returns N(t, x, y, z)
  for Split-Step.
  vEnvelopes are assumed to be given in space-time (t, x, y) or
  (t, r)