1D Linear BPM

FFT-based Beam Propagation Method for linear operators.

This is a simple FFT based beam propagation method for potential-free 1+1+1D Schrodinger equation. The software if flexible and allows to introduce high-order derivatives in the term (very handful if is needed to include dispersion effects). For example, if one desires to solve an equation in the form:

(d/dz + d^2/dx^2 - 0.25*d^3/dx^3)Y == 0

It can be solved using the following code:
z=linspace(0,1,512);
x=linspace(-5,5,1024);
psi0=exp(-x.^2);
D=D_lateral1D(x,2)-0.25*D_lateral1D(x,3);
field=LinearBPM(psi0,D,z);

Where D_lateral1D is a provided function that "replaces" the derivative operator.

This function is a very early stage in a more complex set of non-linear BPM functions that I have made (split-step, spectral methods, etc.) and I will submit them when I finish their documentation and organization.

Sorry for my English, I'm eager for your comments.