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