tk/Convergence_tree.leaves: leave it be more general to be useful for more tree variants

lib/tk/convergence_tree.mli

@@ -22,7 +22,9 @@ val to_newick_tree : t -> Newick.tree
 
 val from_file : string -> (t, [> `Msg of string]) result
 
-val leaves : t -> (string * condition) list
+val leaves :
+  (_, 'l, branch_info) Tree.t ->
+  ('l * condition) list
 
 val infer_binary_condition_on_branches :
   ?gain_relative_cost:float ->