BUG: multiply extremes - logical operators on None objects

The following error is raised if self.label is None

Traceback (most recent call last):
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/run.py", line 29, in main
    PreProcessing()
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/run_modules/pre_processing.py", line 34, in __init__
    self._run()
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/run_modules/pre_processing.py", line 42, in _run
    self.split_train_val_test()
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/run_modules/pre_processing.py", line 72, in split_train_val_test
    self.create_set_split(ind, scope)
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/run_modules/pre_processing.py", line 111, in create_set_split
    set_stations = self.check_valid_stations(args, kwargs, set_stations, load_tmp=False)
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/run_modules/pre_processing.py", line 142, in check_valid_stations
    save_local_tmp_storage=save_tmp)
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/data_handling/data_generator.py", line 197, in get_data_generator
    data.multiply_extremes(self.extreme_values, **kwargs)
  File "/home/f.kleinert/PycharmProjects/machinelearningtools/src/data_handling/data_preparation.py", line 464, in multiply_extremes
    extreme_label_idx = np.concatenate(((self.label < -extr_val).any(axis=0).values.reshape(-1, 1),
TypeError: '<' not supported between instances of 'NoneType' and 'float'
2020-03-23 09:12:10,465 - INFO: PreProcessing finished after 0:22:04 (hh:mm:ss)  [run_environment.py:__del__:39]

Before multiplying extremes, there should be a check if self.labels is not None