STYLE: Remove this->MakeOutput(0) calls from constructors in Core

In those cases, `MakeOutput(0)` just did `OutputType::New()` anyway. This commit
avoids unnecessary casts and calls to virtual functions.

Following C++ Core Guidelines, February 15, 2024, "Don’t call virtual functions
in constructors and destructors",
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-ctor-virtual

Author: N-Dekker

Modules/Core/Common/include/itkImageSource.hxx

@@ -39,11 +39,10 @@ namespace itk
 template <typename TOutputImage>
 ImageSource<TOutputImage>::ImageSource()
 {
-  // Create the output. We use static_cast<> here because we know the default
-  // output must be of type TOutputImage
-  typename TOutputImage::Pointer output = static_cast<TOutputImage *>(this->MakeOutput(0).GetPointer());
   this->ProcessObject::SetNumberOfRequiredOutputs(1);
-  this->ProcessObject::SetNthOutput(0, output.GetPointer());
+
+  // Equivalent to SetNthOutput(0, MakeOutput(0)); in this case, calling MakeOutput is not necessary.
+  this->ProcessObject::SetNthOutput(0, TOutputImage::New());
 
 #if defined(ITKV4_COMPATIBILITY)
   m_DynamicMultiThreading = false;

Modules/Core/Mesh/include/itkImageToMeshFilter.hxx

@@ -27,11 +27,10 @@ template <typename TInputImage, typename TOutputMesh>
 ImageToMeshFilter<TInputImage, TOutputMesh>::ImageToMeshFilter()
 {
   this->ProcessObject::SetNumberOfRequiredInputs(1);
-
-  OutputMeshPointer output = dynamic_cast<OutputMeshType *>(this->MakeOutput(0).GetPointer());
-
   this->ProcessObject::SetNumberOfRequiredOutputs(1);
-  this->ProcessObject::SetNthOutput(0, output.GetPointer());
+
+  // Equivalent to SetNthOutput(0, MakeOutput(0)); in this case, calling MakeOutput is not necessary.
+  this->ProcessObject::SetNthOutput(0, OutputMeshType::New());
 }
 
 /**

Modules/Core/Mesh/include/itkMeshSource.hxx

@@ -25,12 +25,10 @@ namespace itk
 template <typename TOutputMesh>
 MeshSource<TOutputMesh>::MeshSource()
 {
-  // Create the output. We use static_cast<> here because we know the default
-  // output must be of type TOutputMesh
-  OutputMeshPointer output = static_cast<TOutputMesh *>(this->MakeOutput(0).GetPointer());
-
   this->ProcessObject::SetNumberOfRequiredOutputs(1);
-  this->ProcessObject::SetNthOutput(0, output.GetPointer());
+
+  // Equivalent to SetNthOutput(0, MakeOutput(0)); in this case, calling MakeOutput is not necessary.
+  this->ProcessObject::SetNthOutput(0, TOutputMesh::New());
 
   m_GenerateDataRegion = 0;
   m_GenerateDataNumberOfRegions = 0;