docs: def butter_filter(cutoff, fs, order=5, btype='low'):
      '''Create a digital butter fileter with cutoff frequency in Hz
  
      Args
      ----
      cutoff: float
          Cutoff frequency where filter should separate signals
      fs: float
          sampling frequency
      btype: str
          Type of filter type to create. 'low' creates a low-frequency filter and
          'high' creates a high-frequency filter (Default 'low).
  
      Returns
      -------
      b: ndarray
          Numerator polynomials of the IIR butter filter
      a: ndarray
          Denominator polynomials of the IIR butter filter
  
      Notes
      -----
      This function was adapted from the following StackOverflow answer:
      http://stackoverflow.com/a/25192640/943773
      '''
      import scipy.signal
  
      nyq = 0.5 * fs
      normal_cutoff = cutoff / nyq
      b, a = scipy.signal.butter(order, normal_cutoff, btype=btype, analog=False)
  
      return b, a