Add FFT examples

[certik]

1D fft:

• radix 2
• radix 2, 3, 5 + driver
  3D fft:
• using the 1D fft

There is FFTPACK implementation of the above, and I have my own more modern Fortran implementation. For the radix 2 there is also a Python/NumPy implementation here:

• https://jakevdp.github.io/blog/2013/08/28/understanding-the-fft/