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.