docstr: This function calculates the union map between segmentation and
  reference image
  
  :return: union map
docstr_tok: ["This","function","calculates","the","union","map","between","segmentation","and","reference","image",":","return",":","union","map"]
code: def __union_map(self):
      """
      This function calculates the union map between segmentation and
      reference image
  
      :return: union map
      """
      self.check_binary()
      return np.logical_or(self.ref, self.seg).astype(float)
  
imports: from __future__ import absolute_import, print_function
  import numpy
  from scipy import ndimage
  from niftynet.utilities.util_common import MorphologyOps, CacheFunctionOutput
label_desc: Use image node_id 7 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.__union_map() and returns: np