ENH: Make index type of VectorContainer SizeValueType by default

By default, assume `ElementIdentifier = SizeValueType`, for an
`itk::VectorContainer<ElementType>`.

Conceptually like:

    template <typename TElementIdentifier = SizeValueType, typename TElement>
    class VectorContainer;

Implemented by moving the old `VectorContainer` definition into the
`itk::detail` namespace, and introducing a new
`itk::VectorContainer<T1, T2>` alias template.

Modules/Core/Common/include/itkVectorContainer.h

@@ -21,11 +21,14 @@
 #include "itkObject.h"
 #include "itkObjectFactory.h"
 
+#include <type_traits> // For is_void_v.
 #include <utility>
 #include <vector>
 
 namespace itk
 {
+namespace detail
+{
 /** \class VectorContainer
  *  \brief Define a front-end to the STL "vector" container that conforms to the
  *         IndexedContainerInterface.
@@ -561,6 +564,22 @@ class ITK_TEMPLATE_EXPORT VectorContainer
     , VectorType(first, last)
   {}
 };
+} // namespace detail
+
+
+/** Alias template, allowing to use `itk::VectorContainer<TElement>` without having to explicitly specify its
+ * `ElementIdentifier` type.
+ *
+ * The template parameters `T1` and `T2` allow specifying the index type and the element type, as follows:
+ *
+ * \tparam T1 The index type OR (when `T2` is `void`) the element type.
+ *
+ * \tparam T2 The element type OR `void`. When `T2` is `void`, the element type is specified by the first template
+ * argument (T1), and the index type will be `SizeValueType`.
+ */
+template <typename T1, typename T2 = void>
+using VectorContainer = detail::VectorContainer<std::conditional_t<std::is_void_v<T2>, SizeValueType, T1>,
+                                                std::conditional_t<std::is_void_v<T2>, T1, T2>>;
 
 
 /** Makes a VectorContainer that has a copy of the specified `std::vector`. */

Modules/Core/Common/include/itkVectorContainer.hxx

@@ -20,7 +20,7 @@
 
 #include "itkNumericTraits.h"
 
-namespace itk
+namespace itk::detail
 {
 
 template <typename TElementIdentifier, typename TElement>
@@ -183,6 +183,6 @@ template <typename TElementIdentifier, typename TElement>
 void
 VectorContainer<TElementIdentifier, TElement>::Squeeze()
 {}
-} // end namespace itk
+} // namespace itk::detail
 
 #endif

Modules/Core/Common/test/itkVectorContainerGTest.cxx

@@ -27,9 +27,9 @@
 using TestedElementIdentifierType = size_t;
 
 // Test template instantiations for various TElement template arguments:
-template class itk::VectorContainer<TestedElementIdentifierType, int>;
-template class itk::VectorContainer<TestedElementIdentifierType, bool>;
-template class itk::VectorContainer<TestedElementIdentifierType, std::string>;
+template class itk::detail::VectorContainer<TestedElementIdentifierType, int>;
+template class itk::detail::VectorContainer<TestedElementIdentifierType, bool>;
+template class itk::detail::VectorContainer<TestedElementIdentifierType, std::string>;
 
 
 namespace