add Butterworth filter to analysis module

[pietropicerno]

Hi Jens, thanks for replying. So you mean to use fft to get the frequency-domain representation of the signal, and then set the power of unwanted frequencies to zero, followed by ifft to recover the filtered data in the time domain? I'm afraid the approach you suggested would introduce latency and can create undesirable ringing. But I'll try to investigate it. Thanks, Pietro