SDO11 — Operations should return Boolean instead of void

Operations should return Boolean instead of void. Operations that request some action to be performed ((eg. subscribe (data: NotificationSubscription) : void)) does not return anything. Because nothing is returned the requesting SDO will not know if the requested action was carried out successfully or not (eg. if the request for subscription was don’t or not). For this reason Boolean should be returned in such operations. Return Boolean instead of void in such operations.