docs: def butter_apply(b, a, data):
      '''Apply filter with filtfilt to allign filtereted data with input
  
      The filter is applied once forward and once backward to give it linear
      phase, using Gustafsson's method to give the same length as the original
      signal.
  
      Args
      ----
      b: ndarray
          Numerator polynomials of the IIR butter filter
      a: ndarray
          Denominator polynomials of the IIR butter filter
  
      Returns
      -------
      x: ndarray
          Filtered data with linear phase
  
      Notes
      -----
      This function was adapted from the following StackOverflow answer:
      http://stackoverflow.com/a/25192640/943773
      '''
      import scipy.signal
  
      return scipy.signal.filtfilt(b, a, data, method='gust')