Use a single 1D FFT library as backend

[Alan Ayala]

For instance, only use cufft as backend. The following installation option currently fails:

cmake -DHeffte_ENABLE_FFTW=OFF -DHeffte_ENABLE_CUDA=ON ..

[Miroslav Stoyanov]

• now we can use either-or fftw or cufft
• note that turning fftw off will also disable the old interface

I think we can close this. Alan, what do you think?

[Miroslav Stoyanov]

I think this has been resolved.