code: def collect_mean(self, fitness_functions):
          
  
          if not self._need_mean_ff:
              return
  
          self._mean_ff_result.append(np.mean(fitness_functions))
docs: !
          @brief Stores average value of fitness function among chromosomes on specific iteration.
          
          @param[in] fitness_functions (float): Average value of fitness functions among chromosomes.