add Butterworth filter to analysis module

[pietropicerno]

Hi Jens, I hope you're still around. So: I followed your suggestion and I used FFT for removing high frequency noise and then I used invFFT for reconstructing the signal back. I did the implementation in Matlab and it worked as I succeded in obtaining something very similar to the result of a Butterworth filter.
Now the problem with Phyphox is that I do not have a "invFFT" module. Are you planning to release such a module for the editor? Is there any experiment I could refer to?
Thanks, Pietro