Fix TGeo unit system to be the ROOT one

Fix the unit system to the ROOT one which was the default before ROOT
v6.18.02. With ROOT v6.18.02 the ROOT team introduced a new unit system
(taken from Geant4) for the geometry manager.
This change was reverted with ROOT v6.25.1.
Before version v6.18.2 and after v6.25.1 the units are correct by default.

Author: fuhlig1

base/steer/FairRun.cxx

@@ -21,7 +21,12 @@
 #include "FairRuntimeDb.h"      // for FairRuntimeDb
 #include "FairTask.h"           // for FairTask
 
-#include <TFile.h>     // for TFile
+#include <RVersion.h>      // for ROOT_VERSION_CODE and ROOT_VERSION
+#include <TFile.h>         // for TFile
+#include <TGeoManager.h>   // for gGeoManager, TGeoManager
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6, 18, 2) && ROOT_VERSION_CODE <= ROOT_VERSION(6, 22, 6)
+#include <TGeoSystemOfUnits.h>   // for TGeoUnit::kTGeoUnits
+#endif
 #include <TList.h>     // for TList
 #include <TObject.h>   // for TObject
 #include <cassert>     // for... well, assert
@@ -50,8 +55,28 @@ FairRun::FairRun(Bool_t isMaster)
         Fatal("FairRun", "Singleton instance already exists.");
         return;
     }
+
     fRunInstance = this;
 
+    // Fix the unit system to the ROOT one which was the default before ROOT v6.18.02.
+    // With ROOT v6.18.02 the ROOT team introduced a new unit system (taken from Geant4)
+    // for the geometry manager. The change was reverted with ROOT v6.25.1.
+    // Unfortunately the way to set the default units to the ROOT one is
+    // different for different ROOT versions such that the code needs some
+    // preprocessor statements.
+    // Before version v6.18.2 and after v6.25.1 the units are correct
+    // by default
+
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6, 18, 2) && ROOT_VERSION_CODE <= ROOT_VERSION(6, 22, 6)
+    TGeoUnit::setUnitType(TGeoUnit::kTGeoUnits);
+#endif
+
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6, 22, 8) && ROOT_VERSION_CODE <= ROOT_VERSION(6, 25, 1)
+    TGeoManager::LockDefaultUnits(false);
+    TGeoManager::SetDefaultUnits(TGeoManager::EDefaultUnits::kRootUnits);
+    TGeoManager::LockDefaultUnits(true);
+#endif
+
     fRootManager = FairRootManager::Instance();
 
     new FairLinkManager();