Connect Flow Elements to Muon container

Hi all (cc @mhodgkin )

This merge request covers new particle flow objects known as Flow Elements (in the updated EDM) and the linkage of these objects to Muons. This MR only couples charged flow elements to muons via track index matching.

Thanks

Matt

Event/xAOD/xAODPFlow/Root/clids.cxx

+/*
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+*/
+
+// Ensure that the  headers for the interface classes get included
+// by some object in this library, so that CLID registration will take
+// place properly.
+
+#include "xAODPFlow/FlowElementContainer.h"
+#include "xAODPFlow/PFOContainer.h"
+#include "xAODPFlow/TrackCaloClusterContainer.h"