STYLE: Removed LinearInterpolateImageFunction::m_Neighbors

LinearInterpolateImageFunction has a private static const data member
'm_Neighbors' that is only used once, within EvaluateUnoptimized.
This commit removes m_Neighbors. Instead, the number of neighbors
is now computed at compile-time, within EvaluateUnoptimized, storing
the value into a local constexpr variable.

This commit may reduce the runtime overhead somewhat, but it's mostly
a style improvement, simplifying the code.

Change-Id: I96649e11a543fc0f3f246e1f5023ebbc855c520e

Modules/Core/ImageFunction/include/itkLinearInterpolateImageFunction.h

@@ -107,9 +107,6 @@ class ITK_TEMPLATE_EXPORT LinearInterpolateImageFunction:
   void PrintSelf(std::ostream & os, Indent indent) const override;
 
 private:
-  /** Number of neighbors used in the interpolation */
-  static const unsigned long m_Neighbors;
-
   struct DispatchBase {};
   template< unsigned int >
   struct Dispatch: public DispatchBase {};

Modules/Core/ImageFunction/include/itkLinearInterpolateImageFunction.hxx

@@ -25,14 +25,6 @@
 
 namespace itk
 {
-
-// Define the number of neighbors
-template< typename TInputImage, typename TCoordRep >
-const unsigned long
-LinearInterpolateImageFunction< TInputImage, TCoordRep >
-::m_Neighbors = 1 << TInputImage::ImageDimension;
-
-
 template< typename TInputImage, typename TCoordRep >
 LinearInterpolateImageFunction< TInputImage, TCoordRep >
 ::LinearInterpolateImageFunction()
@@ -80,7 +72,10 @@ LinearInterpolateImageFunction< TInputImage, TCoordRep >
 
   Concept::Detail::UniqueType< typename NumericTraits< InputPixelType >::ScalarRealType >();
 
-  for ( unsigned int counter = 0; counter < m_Neighbors; ++counter )
+  // Number of neighbors used in the interpolation
+  constexpr unsigned long numberOfNeighbors = 1 << TInputImage::ImageDimension;
+
+  for ( unsigned int counter = 0; counter < numberOfNeighbors; ++counter )
     {
     InternalComputationType overlap = 1.0;    // Fraction overlap
     unsigned int upper = counter;  // Each bit indicates upper/lower neighbour