class_docstr: simple test class, replace ImageWindowDataset's layer_op
code: def layer_op(self):
      for idx in range(self.n_subjects):
          yield super(ImageWindowGenerator, self).layer_op(idx)
  
imports: from __future__ import absolute_import, print_function
  import os
  import unittest
  import numpy
  import tensorflow
  from niftynet.engine.image_window import N_SPATIAL, LOCATION_FORMAT
  from niftynet.engine.image_window_dataset import ImageWindowDataset
  from niftynet.io.image_reader import ImageReader
  from tests.niftynet_testcase import NiftyNetTestCase
label_desc: Use image node_id 2 for calling the ImageWindowGenerator obj's underlying member method code with example usage: obj.layer_op() without return types