Spectral vorticity calculation

[ivan-pi]

For post-processing, for the periodic case we can calculate the vorticity using a spectral approach. This will require linking with an FFT library:

• FFTW: https://www.fftw.org/
• FFTPACK: https://github.com/fortran-lang/fftpack
• pocketfft: https://github.com/ivan-pi/pocketfft-f