butterworth - simple filters for noisy data

This crate implements digital Butterworth filters for time series data. Filters of arbitrary order can be created and applied to data. Lowpass, highpass, bandpass, and bandstop filters are supported.

Transfer function creation is primarily based on scipy.signal.butter. The bidirectional filter function is designed to match the behavior of MATLAB's filtfilt function. If the default signal padding behavior of used by SciPy's filtfilt is desired, the bidirectional_with_padding function can be used with a padding length of 3 * (filter.order() + 1).