<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def assign_power_curve(self, weather_df):
 r""" Calculates the power curve of the wind turbine cluster. The power curve is aggregated from the wind farms' and wind turbines' power curves by using :func:`power_plant.assign_power_curve`. Depending on the parameters of the WindTurbineCluster power curves are smoothed and/or wake losses are taken into account. Parameters weather_df : pandas.DataFrame DataFrame with time series for wind speed `wind_speed` in m/s, and roughness length `roughness_length` in m, as well as optionally temperature `temperature` in K, pressure `pressure` in Pa, density `density` in kg/m³ and turbulence intensity `turbulence_intensity` depending on `power_output_model`, `density_model` and `standard_deviation_model` chosen. The columns of the DataFrame are a MultiIndex where the first level contains the variable name (e.g. wind_speed) and the second level contains the height at which it applies (e.g. 10, if it was measured at a height of 10 m). See documentation of :func:`TurbineClusterModelChain.run_model` for an example on how to create the weather_df DataFrame. Returns ------- self """


        # Set turbulence intensity for assigning power curve
        turbulence_intensity = (
            weather_df['turbulence_intensity'].values.mean() if
            'turbulence_intensity' in
            weather_df.columns.get_level_values(0) else None)
        # Assign power curve
        if (self.wake_losses_model == 'power_efficiency_curve' or
                self.wake_losses_model == 'constant_efficiency' or
                self.wake_losses_model is None):
            wake_losses_model_to_power_curve = self.wake_losses_model
            if self.wake_losses_model is None:
                logging.debug('Wake losses in wind farms not considered.')
            else:
                logging.debug('Wake losses considered with {}.'.format(
                    self.wake_losses_model))
        else:
            logging.debug('Wake losses considered by {} wind '.format(
                self.wake_losses_model) + 'efficiency curve.')
            wake_losses_model_to_power_curve = None
        self.power_plant.assign_power_curve(
            wake_losses_model=wake_losses_model_to_power_curve,
            smoothing=self.smoothing, block_width=self.block_width,
            standard_deviation_method=self.standard_deviation_method,
            smoothing_order=self.smoothing_order,
            roughness_length=weather_df['roughness_length'][0].mean(),
            turbulence_intensity=turbulence_intensity)
        # Further logging messages
        if self.smoothing is None:
            logging.debug('Aggregated power curve not smoothed.')
        else:
            logging.debug('Aggregated power curve smoothed by method: ' +
                          self.standard_deviation_method)

        return self