fix(sim): Change and move 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()`,
which in turn is called by `FairModule::AddSensitiveVolume()`.

Since the `FairVolumeList::addVolume()` returns nullpointer
in such cases, the commit moves `LOG` and changes it severity
from `LOG(error)` in `FairVolumeList::addVolume()`
to `LOG(debug)` in `FairModule::AddSensitiveVolume()`.

Fixes the issue #1595.

Author: karabowi

fairroot/base/sim/FairModule.cxx

@@ -282,6 +282,8 @@ void FairModule::AddSensitiveVolume(TGeoVolume* vol)
 
     auto addedVol = vList->addVolume(std::make_unique<FairVolume>(volName, fNbOfVolumes));
     if (!addedVol) {
+        LOG(debug) << "FairVolumeList element: " << vol->GetName() << " VolId : " << vol->getVolumeId()
+                   << " already defined " << vol_found->getVolumeId() << "// vol = " << vol;
         return;
     }
     ++fNbOfVolumes;

fairroot/base/sim/FairVolumeList.cxx

@@ -35,8 +35,6 @@ 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()
-                   << " already defined " << vol_found->getVolumeId();
         return nullptr;
     }