RootCollection: Fix cppcheck warnings.

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

Database/APR/RootCollection/src/RootCollectionSchemaEditor.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 "CollectionBase/CollectionBaseNames.h"
@@ -222,7 +222,7 @@ dropIndex( const std::vector<std::string>& /* columnNames  */)
 
 void
 pool::RootCollection::RootCollectionSchemaEditor::
-setUniqueConstraint( std::string /* constraintName */, const std::string& /* columnName  */)
+setUniqueConstraint( const std::string& /* constraintName */, const std::string& /* columnName  */)
 {
    methodNotImplemented("setUniqueConstraint ");
 }
@@ -233,7 +233,7 @@ setUniqueConstraint( std::string /* constraintName */, const std::string& /* col
 
 void
 pool::RootCollection::RootCollectionSchemaEditor::
-setUniqueConstraint( std::string /* constraintName */, const std::vector<std::string>& /* columnNames  */)
+setUniqueConstraint( const std::string& /* constraintName */, const std::vector<std::string>& /* columnNames  */)
 {
    methodNotImplemented("setUniqueConstraint ");
 }

Database/APR/RootCollection/src/RootCollectionSchemaEditor.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 ROOTCOLLECTION_COLLECTIONSCHEMAEDITOR_H
@@ -196,7 +196,7 @@ namespace pool {
          *
          * @param columnName Name of column for which constraint is applied.
          */
-        virtual void setUniqueConstraint( std::string constraintName, const std::string& columnName );
+        virtual void setUniqueConstraint( const std::string& constraintName, const std::string& columnName );
 
         /**
          * Sets a unique constraint on one or more columns of the collection. Automatically generates a
@@ -204,7 +204,7 @@ namespace pool {
          *
          * @param columnNames Names of columns for which constraint is applied.
          */
-        virtual void setUniqueConstraint( std::string constraintName, const std::vector<std::string>& columnNames );
+        virtual void setUniqueConstraint( const std::string& constraintName, const std::vector<std::string>& columnNames );
 
         /**
          * Unsets an existing unique constraint on a single column of the collection,