ENH: Add DataObject::UpdateSource() alternative to GetSource()->Update()

Offers a more efficient, more concise alternative to the following common pattern:

    if (x->GetSource())
    {
      x->GetSource()->Update();
    }

Calling `DataObject::UpdateSource()` instead is more efficient, because it
internally calls `DataObject::GetSource()` just once, instead of twice.

Modules/Core/Common/include/itkDataObject.h

@@ -333,6 +333,10 @@ class ITK_FORCE_EXPORT_MACRO(ITKCommon) DataObject : public Object
   DataObjectPointerArraySizeType
   GetSourceOutputIndex() const;
 
+  /** Does an Update() of its source, if it has one. Does nothing, otherwise. */
+  void
+  UpdateSource() const;
+
   /** Restore the data object to its initial state. This means releasing
    * memory. */
   virtual void

Modules/Core/Common/src/itkDataObject.cxx

@@ -268,6 +268,17 @@ DataObject::GetSourceOutputIndex() const
   return m_Source->MakeIndexFromOutputName(m_SourceOutputName);
 }
 
+void
+DataObject::UpdateSource() const
+{
+  const auto source = this->GetSource();
+
+  if (source)
+  {
+    source->Update();
+  }
+}
+
 //----------------------------------------------------------------------------
 void
 DataObject::PrintSelf(std::ostream & os, Indent indent) const