code: def convert_data_to_ndarray(self):
          
          if self._data_structure != "DataFrame":
              raise Exception(f"Data is not a DataFrame but {self._data_structure}.")
          self._data = self._convert_to_ndarray(self._data)
          self._update_data_structure()
          return self
docs: Converts the data from dataframe to ndarray format. Assumption: df-columns are ndarray-layers (3rd dim.)