support multidimensional transforms

[daanhb]

We currently have:

julia> using GenericFFT

julia> x = rand(BigFloat, 16, 16);

julia> fft(x)
ERROR: MethodError: no method matching generic_fft(::Matrix{Complex{BigFloat}}, ::UnitRange{Int64})

The call fft(x,1) returns something, but fft(x,2) returns an error.

[mfherbst]

Hi @daanhb I appreciate the effort. On the DFTK side we have some hackish code to make this work at least for 3D (2D is similar). I'm lacking the time to give this a serious try at the moment, but happy for anyone to pick the code up if it helps.