Support combining the signals to increase precision

[olivecha]

Possibly :

• Average signals Envelop-wise (test for precision and impact on data)
• Combine trough FFT (loosing precision around the onset)

Create a function such that : Sound = f(Sound1, Sound2, ..., Soundn)

[olivecha]

Added average_envelop_plot() as a global function, computed with an average of the different envelopes with a filter to remove outliers