Tracks backend

Adding TrackBackend in Event/xAOD/xAODTracking and a corresponding test. Tagging @pagessin @cvarni @ncalace @tbold

Event/xAOD/xAODTracking/Root/dict/ContainerProxies.cxx

@@ -16,6 +16,7 @@
 #include "xAODTracking/versions/TrackParametersContainer_v1.h"
 #include "xAODTracking/versions/TrackJacobianContainer_v1.h"
 #include "xAODTracking/versions/TrackMeasurementContainer_v1.h"
+#include "xAODTracking/versions/TrackBackendContainer_v1.h"
 
 // Set up the collection proxies:
 ADD_NS_DV_PROXY( xAOD, TrackParticleContainer_v1 );
@@ -28,3 +29,4 @@ ADD_NS_DV_PROXY( xAOD, TrackStateContainer_v1 );
 ADD_NS_DV_PROXY( xAOD, TrackParametersContainer_v1 );
 ADD_NS_DV_PROXY( xAOD, TrackJacobianContainer_v1 );
 ADD_NS_DV_PROXY( xAOD, TrackMeasurementContainer_v1 );
+ADD_NS_DV_PROXY( xAOD, TrackBackendContainer_v1 );