Starts the scheduler in a new thread. Returns 0 if success.
        In standalone mode, this method will block until there are no more scheduled jobs.

def start(self):
        """
        Starts the scheduler in a new thread. Returns 0 if success.
        In standalone mode, this method will block until there are no more scheduled jobs.
        """
        self.history.append("Started on %s" % time.asctime())
        self.start_time = time.time()

        if not has_apscheduler:
            raise RuntimeError("Install apscheduler with pip")

        if has_sched_v3:
            self.sched.add_job(self.callback, "interval", **self.sched_options)
        else:
            self.sched.add_interval_job(self.callback, **self.sched_options)

        errors = self.flow.look_before_you_leap()
        if errors:
            self.exceptions.append(errors)
            return 1

        # Try to run the job immediately. If something goes wrong return without initializing the scheduler.
        self._runem_all()

        if self.exceptions:
            self.cleanup()
            self.send_email(msg="Error while trying to run the flow for the first time!\n %s" % self.exceptions)
            return 1

        try:
            self.sched.start()
            return 0

        except KeyboardInterrupt:
            self.shutdown(msg="KeyboardInterrupt from user")
            if ask_yesno("Do you want to cancel all the jobs in the queue? [Y/n]"):
                print("Number of jobs cancelled:", self.flow.cancel())

            self.flow.pickle_dump()
            return -1