STYLE: Address Greptile review on HigherOrderAccurateGradient

- Drop `== true` boolean comparisons (itkHigherOrderAccurateDerivativeImageFilter.hxx,
  itkHigherOrderAccurateGradientImageFilter.hxx).
- Collapse stray blank line between `Ctor()` signature and `= default;` in
  Derivative filter, GradientImageFilter ctor, and DerivativeOperator ctor.
- Document SetOrder() limitation: only `order == 1` is implemented;
  GenerateCoefficients() throws on any other value.
- Rename misnamed wrap file
  itkHigherOrderAccurateGradientmageFilter.wrap ->
  itkHigherOrderAccurateGradientImageFilter.wrap.

Author: hjmjohnson

Modules/Filtering/HigherOrderAccurateGradient/include/itkHigherOrderAccurateDerivativeImageFilter.h

@@ -115,9 +115,7 @@ class HigherOrderAccurateDerivativeImageFilter : public ImageToImageFilter<TInpu
   itkGetConstMacro(UseImageSpacing, bool);
 
 protected:
-  HigherOrderAccurateDerivativeImageFilter()
-
-    = default;
+  HigherOrderAccurateDerivativeImageFilter() = default;
 
   ~HigherOrderAccurateDerivativeImageFilter() override = default;
   void

Modules/Filtering/HigherOrderAccurateGradient/include/itkHigherOrderAccurateDerivativeImageFilter.hxx

@@ -99,7 +99,7 @@ HigherOrderAccurateDerivativeImageFilter<TInputImage, TOutputImage>::GenerateDat
   oper.CreateDirectional();
   oper.FlipAxes();
 
-  if (m_UseImageSpacing == true)
+  if (m_UseImageSpacing)
   {
     if (this->GetInput()->GetSpacing()[m_Direction] == 0.0)
     {

Modules/Filtering/HigherOrderAccurateGradient/include/itkHigherOrderAccurateDerivativeOperator.h

@@ -72,9 +72,7 @@ class HigherOrderAccurateDerivativeOperator : public NeighborhoodOperator<TPixel
   itkTypeMacro(HigherOrderAccurateDerivativeOperator, NeighborhoodOperator);
 
   /** Constructor. */
-  HigherOrderAccurateDerivativeOperator()
-
-    = default;
+  HigherOrderAccurateDerivativeOperator() = default;
 
   /** Copy constructor. */
   HigherOrderAccurateDerivativeOperator(const Self & other)
@@ -92,7 +90,9 @@ class HigherOrderAccurateDerivativeOperator : public NeighborhoodOperator<TPixel
     return *this;
   }
 
-  /** Sets the order of the derivative. */
+  /** Sets the order of the derivative. Only `order == 1` is currently
+   * implemented; any other value triggers a runtime exception in
+   * GenerateCoefficients(). */
   void
   SetOrder(const unsigned int & order)
   {

Modules/Filtering/HigherOrderAccurateGradient/include/itkHigherOrderAccurateGradientImageFilter.hxx

@@ -30,9 +30,7 @@ namespace itk
 
 template <typename TInputImage, typename TOperatorValueType, typename TOutputValueType>
 HigherOrderAccurateGradientImageFilter<TInputImage, TOperatorValueType, TOutputValueType>::
-  HigherOrderAccurateGradientImageFilter()
-
-  = default;
+  HigherOrderAccurateGradientImageFilter() = default;
 
 
 template <typename TInputImage, typename TOperatorValueType, typename TOutputValueType>
@@ -126,7 +124,7 @@ HigherOrderAccurateGradientImageFilter<TInputImage, TOperatorValueType, TOutputV
     op[i].FlipAxes();
 
     // Take into account the pixel spacing if necessary
-    if (m_UseImageSpacing == true)
+    if (m_UseImageSpacing)
     {
       if (this->GetInput()->GetSpacing()[i] == 0.0)
       {