docstr: This function calculates the centre of mass of the reference
  segmentation
  
  :return:
docstr_tok: ["This","function","calculates","the","centre","of","mass","of","the","reference","segmentation",":","return",":"]
code: def com_ref(self):
      """
      This function calculates the centre of mass of the reference
      segmentation
  
      :return:
      """
      return ndimage.center_of_mass(self.ref) * np.array(self.pixdim)
  
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 31 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.com_ref() and returns: unknown