Add FNodeIteratorBox constainer interface for FTree walks

FNodeIteratorBox is a dummy container that is used by the tree to offer
begin and end iterator according to the chosen walk order.

Src/Adaptive/new/FNodeIteratorBox.hpp

+#ifndef _SCALFMM_NODE_ITERATOR_BOX_
+#define _SCALFMM_NODE_ITERATOR_BOX_
+
+#include <iterator>
+
+template <class Node, template <class> class NodeIterator>
+class FNodeIteratorBox {
+    using node_t = Node;
+public:
+
+    using value_type      = node_t;
+    using reference       = node_t&;
+    using const_reference = const node_t&;
+    using iterator        = NodeIterator<node_t>;
+    using const_iterator  = NodeIterator<const node_t>;
+    using difference_type = typename iterator::difference_type;
+    using size_type = std::size_t;
+
+
+    node_t* _root = nullptr;
+
+    FNodeIteratorBox(node_t* root) : _root(root) { }
+
+    iterator begin() {
+        return iterator(_root);
+    }
+
+    iterator end() {
+        return iterator(_root, iterator::IteratorPosition::end);
+    }
+
+};
+
+#endif