fix(sim): Change log level for multiple sensitive volumes with same name

Both the transport engines and FairRoot allow registering
of multiple nodes with the same volume/same volume name (copy mechanism).
However currently such workflow logs errors from `FairVolumeList::addVolume()`.

The commit changes the LOG(error) to LOG(debug).

Fixes the issue #1595.

Author: karabowi

fairroot/base/sim/FairVolumeList.cxx

@@ -35,7 +35,7 @@ FairVolume* FairVolumeList::addVolume(std::unique_ptr<FairVolume> vol)
     auto vol_found = findObject(vol->GetName());
 
     if (vol_found) {
-        LOG(error) << "FairVolumeList element: " << vol->GetName() << " VolId : " << vol->getVolumeId()
+        LOG(debug) << "FairVolumeList element: " << vol->GetName() << " VolId : " << vol->getVolumeId()
                    << " already defined " << vol_found->getVolumeId();
         return nullptr;
     }