Draft: StoreGate: WriteDecorHandle consistency check.

Add a check to WriteDecorHandle that the provided AuxElement is actually a member of the declared container.

(For now, don't do the same thing for ReadDecorHandle, as that would add extra overhead.)

Control/StoreGate/StoreGate/ReadDecorHandle.icc

 /*
- * Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration.
+ * Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration.
  */
 /**
  * @file StoreGate/ReadDecorHandle.icc
@@ -128,6 +128,9 @@ inline
 typename ReadDecorHandle<T, D>::const_reference_type
 ReadDecorHandle<T, D>::operator() (const AuxElement& e) const
 {
+  // FIXME?  In principle, should check here that E is actually an element
+  // of our declared container.  But that would force a SG lookup here
+  // which we otherwise wouldn't need to do.
   return m_acc (e);
 }