message: Using decayer in Init, when set
  if not set, use external decayer from TVirtualMC::GetMC() (as before)
  if no decayer available, return Fatal
  needed for adding Exodus decayer for single AliGenParam sources in cocktail
diff: @@ -438,7 +438,12 @@ void AliGenParam::Init()
  {
  // Initialisation
  - if (TVirtualMC::GetMC()) fDecayer = TVirtualMC::GetMC()->GetDecayer();
  + if (!fDecayer && TVirtualMC::GetMC()) fDecayer = TVirtualMC::GetMC()->GetDecayer();
  + if (!fDecayer){
  + Fatal("AliGenParam",
  + "No decayer attached");
  + }
  +
  //Begin_Html
  /*
  <img src="picts/AliGenParam.gif">
  