Generate assertion on _self <> null

[tongbong]

K3AL should always check _self against null. It is a nightmare to debug otherwise because xtend will enter the method even when called on a null object.

  public static void accept(final FSMClock _self, final IVisitor visitor) {
    _self_ = org.gemoc.sample.tfsm.purek3.dsa.FSMClockAspectFSMClockAspectContext.getSelf(_self);
   // HERE assert.isNotNull(_self)
    Object result = null;
    _privk3_accept(_self,visitor);          
  }