fft_plot function

Plot signal FFT norm and phase versus frequency and signal versus time. Customizable display, also return FFT data.

Function can also return fft results.

optionnal input argument:
- 'legend': 'signal name'
- 'title': 'your title'
- 'window': 'none', 'hann', 'hamming', 'flattop', 'blackman', 'blackmanharris'
- 'subplot': 'fft', 'fft_signal', 'fft_angle', 'all', 'none'
- 'MinNorm': if norm is lower than this value, angle is forced to 0.
(angle makes no sense for small norm)
- 'H1': replace frequency axis with harmonic number (divide frequency axis by H1)
- 'fmin': display frequency fmin to fs/2 (if H1 != 1 then fmin = Harmonic min)
- 'fmax': display frequency 0 to fmax (if H1 != 1 then fmax = Harmonic max)
- 'Amax': display amplitude 0 to Amax
- 'style': 'line', 'stem', 'bar', 'stairs', 'area', (fft norm only)
- 'scale': 'linear', 'semilogx', 'semilogy', 'loglog'
- 'linewidth': number (defaut is 0.5)