Complete Redesign

This projects definitely needs a redesign. It should be CUDA compatible and also differentiable by automatic differentiation!

First, I want to focus (:zany_face:) on scalar optics.

I think it might make sense to introduce types like:
```julia
struct ScalarField{T, N, A <: Union{Array{T, N}, CuArray{T, N}}}
# wavelength
λ :: T
# length of eletrical field
L :: T
# Array, CuArray, ... of
field :: A
end
```

Those are the trivial parameters we need to describe a field. Scalar Fourier optics is possible with such a type.

The issue names no files, tests, or entry points. Start by reading the existing Julia package structure and determine how scalar optics is currently represented, then assess the proposed ScalarField type against the CUDA and automatic-differentiation goals. Done would require an agreed design and an implemented scalar-optics foundation, but the scope is not defined here.

Written by the indexing model from the issue text.