message: Added slight tikhonov regularization in GetNewtonUpdate in order to
  handle null hessians
diff: @@ -105,9 +105,11 @@ GetNewtonUpdateImageFilter< TGradient, THessian>
  {
  // Make a vnl matrix out of the values read in input 2 (the hessian, but stored in a vector)
  vnl_matrix<dataType> hessian = vnl_matrix<dataType>(hessIt.Get().GetDataPointer(), nChannels, nChannels);
  + vnl_matrix<dataType> regul = vnl_matrix<dataType>(nChannels, nChannels, 0);
  + regul.fill_diagonal(1e-8);
  // Invert the hessian, multiply by the gradient, and write it in output
  - forOutput.SetVnlVector(vnl_inverse(hessian) * gradIt.Get().GetVnlVector());
  + forOutput.SetVnlVector(vnl_inverse(hessian + regul) * gradIt.Get().GetVnlVector());
  outIt.Set(forOutput);
  ++outIt;
  