ENH: Add cbegin() and cend() member functions to Index, Offset, Size

Mirrored the corresponding member functions of `std::array`.

Following commit 549a19c "STYLE: Change aggregate classes to mirror std::array", Hans Johnson, 2018-02-12

Author: N-Dekker

Modules/Core/Common/include/itkIndex.h

@@ -346,6 +346,12 @@ struct ITK_TEMPLATE_EXPORT Index final
     std::swap(m_InternalArray, other.m_InternalArray);
   }
 
+  const_iterator
+  cbegin() const
+  {
+    return &m_InternalArray[0];
+  }
+
   iterator
   begin()
   {
@@ -358,6 +364,12 @@ struct ITK_TEMPLATE_EXPORT Index final
     return &m_InternalArray[0];
   }
 
+  const_iterator
+  cend() const
+  {
+    return &m_InternalArray[VDimension];
+  }
+
   iterator
   end()
   {

Modules/Core/Common/include/itkOffset.h

@@ -299,6 +299,12 @@ struct ITK_TEMPLATE_EXPORT Offset final
     std::swap(m_InternalArray, other.m_InternalArray);
   }
 
+  const_iterator
+  cbegin() const
+  {
+    return &m_InternalArray[0];
+  }
+
   iterator
   begin()
   {
@@ -311,6 +317,12 @@ struct ITK_TEMPLATE_EXPORT Offset final
     return &m_InternalArray[0];
   }
 
+  const_iterator
+  cend() const
+  {
+    return &m_InternalArray[VDimension];
+  }
+
   iterator
   end()
   {

Modules/Core/Common/include/itkSize.h

@@ -258,6 +258,12 @@ struct ITK_TEMPLATE_EXPORT Size final
     std::swap(m_InternalArray, other.m_InternalArray);
   }
 
+  const_iterator
+  cbegin() const
+  {
+    return &m_InternalArray[0];
+  }
+
   iterator
   begin()
   {
@@ -270,6 +276,12 @@ struct ITK_TEMPLATE_EXPORT Size final
     return &m_InternalArray[0];
   }
 
+  const_iterator
+  cend() const
+  {
+    return &m_InternalArray[VDimension];
+  }
+
   iterator
   end()
   {