nonlinear part can be used by spli-step
  dA(omega, kx, ky)/dz = L(omega, kx, ky)A + N(omega, kx, ky)
  this fnction returns N(t, x, y) - it does Multiple Fourier 
  transforms to account for steepening and kz in the
  denominator
 
  caNonlinearPart is ca of N for all envelopes.