CExpEuler First-order forward Euler exponential integrator
    in future will become special case of ExpRK
    this solves problems like dA/dz = L A + N
    so at each step L and N is required.
 
  good for linear propagation, spm (gets slow, but acurate)
  for nonaxial propagation requires acuracy < 1e-4 forcing
  unrealistically small steps.