ImplicitCollection: Fix cppcheck warnings.

 - Pass class instances by const reference, not by value.

Database/APR/ImplicitCollection/src/ImplicitCollection.cpp

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "ImplicitCollection.h"
@@ -24,8 +24,8 @@ namespace pool {
 
    ImplicitCollection::
    ImplicitCollection( ISession* session,
-                       std::string connection,
-                       std::string name,
+                       const std::string& connection,
+                       const std::string& name,
                        ICollection::OpenMode mode )
          :
          m_container( 0 ),

Database/APR/ImplicitCollection/src/ImplicitCollection.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef INCLUDE_IMPLICITCOLLECTION_IMPLICITCOLLECTION_H
@@ -36,8 +36,8 @@ namespace pool {
        @param mode collection's open mode. For the moment only READONLY mode is allowed.
     */
     ImplicitCollection( ISession* session,
-                        std::string connection,
-                        std::string name,
+                        const std::string& connection,
+                        const std::string& name,
                         ICollection::OpenMode mode );
 
     /// Constructor compying to the new Collections API