code: def preprocess(self, data_group):
      for file_idx, filename in enumerate(data_group.preprocessed_case):
          specific_command = self.command + [
              "N4ITKBiasFieldCorrection",
              quotes(filename),
              quotes(self.output_filenames[file_idx]),
          ]
          subprocess.call(
              " ".join(specific_command),
              shell=True,
              stdout=FNULL,
              stderr=subprocess.STDOUT,
          )
  
      self.output_data = self.output_filenames
      data_group.preprocessed_case = self.output_filenames
  
imports: import subprocess
  import os
  import numpy
  from deepneuro.preprocessing.preprocessor import Preprocessor
  from deepneuro.utilities.conversion import read_image_files
  from deepneuro.utilities.util import add_parameter, quotes
label_desc: Use image node_id 2 for calling the N4BiasCorrection obj's underlying member method code with example usage: obj.preprocess(data_group) without return types