Use of generic FFT and cuFFT library on CPUs/GPUs in x3div

[pbartholomew08]

To make sure that x3div can use the generic FFT (no need for external librairies) on CPU/GPU, and the cuFFT on NVIDIA GPUs. Default setting should be FFT generic option.

It should be noted that the generic FFT currently need explicit Openacc code to run on NVIDIA GPUs. We need an update on this from NVIDIA. See other issue raised for this problem with openacc (we want to avoid openacc and rely only on do concurrent.

Discussion about FFTW: do we need to have this option available?