GPU support for steadystate_fourier

[albertomercurio]

The steadystate_fourier function doesn't support GPU arrays.

The support can be implemented by following the same method as steadystate.

One can take advantage of the following functions

• _dense_similar: Creates the equivalent dense version of the input array. Uses multiple dispatch to generate a GPU array if necessary.
• allowed_setindex!: Allows to set the index of an array. Use this only if the operation is done only once.

[aarontrowbridge]

if it makes sense, which it looks like it does, to use CUDSS.jl in LinearSolve.jl to do this I'm happy to take a wack at it this weekend

i didn't look at the work @Fe-r-oz did already to see if this approach makes sense or not

[albertomercurio]

Hi @aarontrowbridge,

As far as I know, CUDSS.jl allows to solve direct methods, right?

Here I think that we should use iterative methods, as the matrix is usually large.

If we make it work with iterative methods (which should be easier that using CUDSS.jl I think), then we can think to implement the direct solver, but maybe in another PR.

You can take the steadystate function as an example, and also the last part of this section.