COMP: Out-of-bounds access GaussianDerivativeImageFunction.

Coverity Scan:

  CID 1081055 (#1 of 1): Out-of-bounds access (OVERRUN)
  41. overrun-call: Overrunning callee's array of size 4 by passing argument
      "op" (which evaluates to 4) in call to
      "itk::FixedArray<itk::Neighborhood<double, 2u,
      itk::NeighborhoodAllocator<double> >, 4u>::operator [](unsigned
      int)".[hide details]

Change-Id: I96715fc7674d7adb7dc0becfa68f2aa541a3fb1a
ENH: Use CompensatedSummation in GaussianDerivativeImageFunction.

Change-Id: Id8c754ce84e9ad7a0a2e3020bb541bd0754faad4
Merge topic 'coverity_gaussian'

3ae9600 ENH: Use CompensatedSummation in GaussianDerivativeImageFunction.
7b5fbef COMP: Out-of-bounds access GaussianDerivativeImageFunction.