Frequency domain corrections for nth octave spectrum and screening for tones functions

[wantysal]

• The function noct_synthesis and noct_freq_filter have been enhanced for better accuracy compared to the filtering in time domain. The function butter and sosfreqz from scipy.signal are used.
• In time domain, the use of a,b filter representation have been replaced by sos to improve the performance of noct_spectrum.
• The function screening_for_tones has been corrected to correctly detect a potential peak on the first index
• The function spectrum_smoothing now takes as input a spectrum of dimension (nperseg, nseg) as the rest of the functions

All the tests are running without any warning.

Tks :)

[mglesser]

Thank you for your contribution