add warning in trace operation

experimental/include/scalfmm/tree/group_tree.hpp

@@ -420,9 +420,12 @@ namespace scalfmm::component
         ///  interaction list) level_trace = 5 print information of the tree (leaf and cell interval and index inside
         ///  and their p2p and m2l interaction lists)
         ///
-        /// \param[in] level_trace level of the trace
+        /// @warning To have the right p2p list we have to have the groupe size in the tree equal to the number
+        ///  of leaves otherwise, we only print the index inside the group.
         ///
-        inline auto trace(std::size_t level_trace = 0) -> void
+        /// @param[in] level_trace level of the trace
+        ///
+        inline auto trace(const std::size_t level_trace = 0) -> void
         {
             auto level_0 = []() {};