fix: raise an error when Algo.diameter is called on a non-connected graph

lib/sasacore/register.ml

-(* Time-stamp: <modified the 21/04/2021 (at 15:58) by Erwan Jahier> *)
+(* Time-stamp: <modified the 27/07/2021 (at 10:36) by Erwan Jahier> *)
 
 type 's neighbor = {
   state:  's ;
@@ -321,6 +321,7 @@ let (links_number : unit -> int) =
 
 let (diameter : unit -> int) =
   fun () ->
+  if not (is_connected()) then failwith "diameter: the graph is not connected";
   match tbls.diameter with
   | Some x -> x
   | None ->