code: def _cosine_function(x, a, b, t_shift):
      
  
      mean_wind, t = x
      return a * mean_wind * np.cos(np.pi * (t - t_shift) / 12) + b * mean_wind
docs: genrates a diurnal course of windspeed accroding to the cosine function
  
      Args:
          x: series of euqally distributed windspeed values
          a: parameter a for the cosine function
          b: parameter b for the cosine function
          t_shift: parameter t_shift for the cosine function
          
      Returns:
          series including diurnal course of windspeed.