the integral of the linear part can be used by spli-step and by
  exponential RK
  dA(omega, kx, ky)/dz = L(omega, kx, ky, z)A + N(omega, kx, ky)
  this fnction returns:
     int_z^{z+\Delta z} L(omega, kx, ky, s) ds
  
  caLinearPartInt is an array of int L for all envelopes.