Return tasks that was started. Result way be filtered by the given arguments.

		:param task_registry_id: if it is specified, then try to return single task which id is the same as \
		this value.
		:param task_cls: if it is specified then result will be consists of this subclass only

		:return: None or WTask or tuple of WTask

def started_tasks(self, task_registry_id=None, task_cls=None):
		""" Return tasks that was started. Result way be filtered by the given arguments.

		:param task_registry_id: if it is specified, then try to return single task which id is the same as \
		this value.
		:param task_cls: if it is specified then result will be consists of this subclass only

		:return: None or WTask or tuple of WTask
		"""

		if task_registry_id is not None:
			task = None
			for registered_task in self.__started:
				if registered_task.__registry_tag__ == task_registry_id:
					task = registered_task

			if task_cls is not None and task is not None:
				if isinstance(task, task_cls) is True:
					return task
				return None
			return task

		result = filter(lambda x: x is not None, self.__started)
		if task_cls is not None:
			result = filter(lambda x: isinstance(x, task_cls), result)
		return tuple(result)