code: public DefaultJobProgressStep nextStep(Message stepMessage, Object newStepSource)
      {
          assertModifiable();
  
          // Close current step and move to the end
          finishStep();
  
          // Add new step
          return addStep(stepMessage, newStepSource);
      }
docs: Move to next child step.
  
  @param stepMessage the message associated to the step
  @param newStepSource who asked to create this new step
  @return the new step