Compliant with the "Teoz" architecture of plantuml for nice print of broadcast

src/main/scala/org/scadcop/solver/decentralized/agent/VariableAgent.scala

@@ -34,10 +34,17 @@ abstract class VariableAgent(val variable: Variable,
     * Multicast a message to some peers
     */
   def multicast(peers : Set[Variable], message : Message): Unit = {
+    var isFirst = true
+    var log = ""
     peers.foreach { other =>
-      if (trace) println(s"$variable-> $other : $message")
+      if (trace){
+        if (isFirst) log += s"$variable-> $other : $message\n"
+        else log += s"& $variable -> $other :\n"
+      }
+      isFirst = false
       directory.addressOf(other) ! message
     }
+    if (trace) print(log)
   }
 
   /**