caNonlinearPartFourierSpace = getNonlinearPartFourierSpace(caNonlinearPartFourierSpace, vEnvelopes, fPropagationCoordinateZ)
  dA(omega, kx, ky)/dz = LA + N(omega, kx, ky)
  this fnction returns N(omega, kx, ky, z)
 
  for Exp and IF RK methods