BUG: Fix race conditions in itkInvertDisplacementFieldImageFilter

In InvertDisplacementFieldImageFilter::ThreadedGenerateData(), multiple threads simultaneously read from and write to member variables m_MeanErrorNorm and m_MaxErrorNorm. These race conditions are fixed with a mutex.

Change-Id: I3bea5406351e66019dbfc3df3bde28566ff02f37
STYLE: Incorporated reviewer suggestions

Change-Id: If6d17b2705fa76eb7b8d470503165c03ad823220
Merge topic 'release-FixRaceConditionsInInvertDisplacementFieldImageFilter'

eb8ae2fb STYLE: Incorporated reviewer suggestions
87458a2b BUG: Fix race conditions in itkInvertDisplacementFieldImageFilter