class_docstr: simple test class, replace ImageWindowDataset's layer_op
code: def __init__(self, *args, **kwargs):
      ImageWindowDataset.__init__(self, *args, **kwargs)
  
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 1 to create a new ImageWindowGenerator object from inherited base classes: ImageWindowDataset with example: obj = ImageWindowGenerator()