Add option to filter signals

[cbrnr]

Some biosignals require filtering before they can be visualized, e.g. EEG recorded without any HP filter typically has huge drifts. The auto-centering function, which centers each channel around its mean, doesn't work in many cases where the variance of the drift is large compared to its mean.

There should be at least two filter types, namely high-pass and low-pass filters. Band-pass and band-stop can be constructed by combining these two types.

We need to think about how we integrate filtering into SigViewer. Do we treat it like auto-centering (that is, something that can be toggled on and off)? Or do we open a new tab? How can multiple filters be applied sequentially? Which parameters does the user set before filtering?