STYLE: Add BoundaryConditionType to GradientImageFilter

Added a helper typedef, for `GradientImageFilter::OverrideBoundaryCondition`
and its implementation.

Author: N-Dekker

Modules/Filtering/ImageGradient/include/itkGradientImageFilter.h

@@ -100,6 +100,9 @@ class ITK_TEMPLATE_EXPORT GradientImageFilter : public ImageToImageFilter<TInput
   using CovariantVectorType = CovariantVector<OutputValueType, Self::OutputImageDimension>;
   using OutputImageRegionType = typename OutputImageType::RegionType;
 
+  /** Helper typedef for the parameter type of OverrideBoundaryCondition. */
+  using BoundaryConditionType = ImageBoundaryCondition<TInputImage>;
+
   /** GradientImageFilter needs a larger input requested region than
    * the output requested region.  As such, GradientImageFilter needs
    * to provide an implementation for GenerateInputRequestedRegion()
@@ -140,7 +143,7 @@ class ITK_TEMPLATE_EXPORT GradientImageFilter : public ImageToImageFilter<TInput
 
   /** Allows to change the default boundary condition */
   void
-  OverrideBoundaryCondition(ImageBoundaryCondition<TInputImage> * boundaryCondition);
+  OverrideBoundaryCondition(BoundaryConditionType * boundaryCondition);
 
 #ifdef ITK_USE_CONCEPT_CHECKING
   // Begin concept checking
@@ -228,7 +231,7 @@ class ITK_TEMPLATE_EXPORT GradientImageFilter : public ImageToImageFilter<TInput
   bool m_UseImageDirection{ true };
 
   // allow setting the m_BoundaryCondition
-  std::unique_ptr<ImageBoundaryCondition<TInputImage, TInputImage>> m_BoundaryCondition{
+  std::unique_ptr<BoundaryConditionType> m_BoundaryCondition{
     std::make_unique<ZeroFluxNeumannBoundaryCondition<TInputImage>>()
   };
 };

Modules/Filtering/ImageGradient/include/itkGradientImageFilter.hxx

@@ -40,7 +40,7 @@ GradientImageFilter<TInputImage, TOperatorValueType, TOutputValueType, TOutputIm
 template <typename TInputImage, typename TOperatorValueType, typename TOutputValue, typename TOutputImage>
 void
 GradientImageFilter<TInputImage, TOperatorValueType, TOutputValue, TOutputImage>::OverrideBoundaryCondition(
-  ImageBoundaryCondition<TInputImage> * boundaryCondition)
+  BoundaryConditionType * boundaryCondition)
 {
   m_BoundaryCondition.reset(boundaryCondition);
 }