[BL] ConfigTree; Fix arguments of xxxIterator ctor

 - The Iterator type is trivial; passing it by const&.
 - The std::string root is invoked only on const& strings, so
moving it does not save anything in this context.

BaseLib/ConfigTree.h

@@ -106,9 +106,9 @@ public:
     public:
         using Iterator = boost::property_tree::ptree::const_assoc_iterator;
 
-        explicit SubtreeIterator(Iterator it, std::string root,
+        explicit SubtreeIterator(Iterator const& it, std::string const& root,
                                  ConfigTree const& parent)
-            : _it(it), _tagname(std::move(root)), _parent(parent)
+            : _it(it), _tagname(root), _parent(parent)
         {}
 
         SubtreeIterator& operator++() {
@@ -180,9 +180,9 @@ public:
     public:
         using Iterator = boost::property_tree::ptree::const_assoc_iterator;
 
-        explicit ValueIterator(Iterator it, std::string root,
+        explicit ValueIterator(Iterator const& it, std::string const& root,
                                ConfigTree const& parent)
-            : _it(it), _tagname(std::move(root)), _parent(parent)
+            : _it(it), _tagname(root), _parent(parent)
         {}
 
         ValueIterator<ValueType>& operator++() {