code: def mse(self):
      return np.mean(np.square(self.reg - self.ref))
  
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 2 for calling the PairwiseMeasuresRegression obj's underlying member method code with example usage: obj.mse() and returns: np