code: def rmse(self):
      return np.sqrt(self.mse())
  
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 3 for calling the PairwiseMeasuresRegression obj's underlying member method code with example usage: obj.rmse() and returns: np