docs: def compstat(sdat, tstart=None, tend=None):
      """Compute statistics from series output by StagYY.
  
      Create a file 'statistics.dat' containing the mean and standard deviation
      of each series on the requested time span.
  
      Args:
          sdat (:class:`~stagpy.stagyydata.StagyyData`): a StagyyData instance.
          tstart (float): starting time. Set to None to start at the beginning of
              available data.
          tend (float): ending time. Set to None to stop at the end of available
              data.
      """
      data = sdat.tseries_between(tstart, tend)
      time = data['t'].values
      delta_time = time[-1] - time[0]
      data = data.iloc[:, 1:].values  # assume t is first column
  
      mean = np.trapz(data, x=time, axis=0) / delta_time
      rms = np.sqrt(np.trapz((data - mean)**2, x=time, axis=0) / delta_time)
  
      with open(misc.out_name('statistics.dat'), 'w') as out_file:
          mean.tofile(out_file, sep=' ', format="%10.5e")
          out_file.write('\n')
          rms.tofile(out_file, sep=' ', format="%10.5e")
          out_file.write('\n')