MagFieldServices: Load dictionaries early.

Root has problems with loading dictionaries if multiple threads are running.
Force load some dictionaries we'll need during initialization.
See ATEAM-749.

MagneticField/MagFieldServices/src/AtlasFieldMapCondAlg.cxx

 /*
-  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 /**
@@ -27,6 +27,7 @@
 // ROOT
 #include "TFile.h"
 #include "TTree.h"
+#include "TClass.h"
 
 MagField::AtlasFieldMapCondAlg::AtlasFieldMapCondAlg(const std::string& name, 
                                                    ISvcLocator* pSvcLocator)
@@ -63,6 +64,14 @@ MagField::AtlasFieldMapCondAlg::initialize() {
     else {
         ATH_MSG_INFO ( "Initialize: Will update the field map from jobOpt file name");
     }
+
+    // Load these dictionaries now, so we don't need to try to do so
+    // while multiple threads are running.
+    TClass::GetClass ("TLeafI");
+    TClass::GetClass ("TLeafD");
+    TClass::GetClass ("TLeafO");
+    TClass::GetClass ("TLeafS");
+
     return StatusCode::SUCCESS;
 }