code: def sigma_fltr(dem, n=3):
      
      std = dem.std()
      u = dem.mean()
      print('Excluding values outside of range: {1:0.2f} +/- {0}*{2:0.2f}'.format(n, u, std))
      rangelim = (u - n*std, u + n*std)
      out = range_fltr(dem, rangelim)
      return out
docs: sigma * factor filter
      
      Useful for outlier removal
  
      These are min/max percentile ranges for different sigma values:
      1: 15.865, 84.135
      2: 2.275, 97.725
      3: 0.135, 99.865