Draft: added object type to iovelement

This just adds

using object_type = ObjectType ;

to DeIOVElement such that the concrete handle type (e.g. DeVeloSensor) knows for which payload type (e.g. DeVPSensorObject) it is a handle.

To be used for Alignment!293 (closed).

Core/include/Core/DeIOV.h

@@ -93,6 +93,7 @@ namespace LHCb::Detector {
   template <typename ObjectType>
   struct DeIOVElement : dd4hep::Handle<ObjectType>, IAlignable {
     using dd4hep::Handle<ObjectType>::Handle;
+    using object_type = ObjectType ;
 
     /// Accessor to the geometry structure of this detector element
     std::string lVolumeName() const override { return placement().volume().name(); }