add notification code

- (RLMNotificationToken *)startCreatingTasksForOperationType:(MUSOperationType)type {
    RLMResults *results = self.resultsBlock(type);

    if (!results) {
        return nil;
    }

    return [results addNotificationBlock:^(RLMResults * _Nullable results, RLMCollectionChange * _Nullable change, NSError * _Nullable error) {
        if (error) {
            NSLog(@"%@ results error : %@", self, error);
            return;
        }

        if (!change) {
            [self processObjects:(id<MUSSubscriptable>)results forOperationType:type];
        } else {
            // Realm converts moves to deletion and insertion pairs : https://github.com/realm/realm-cocoa/issues/3571
            // We only want added or modified objects, so we ignore any moved or deleted objects. (moved == delete/insert pair)

            NSMutableSet<NSNumber *> *insertionsWithoutMoves = [NSMutableSet setWithArray:change.insertions];
            [insertionsWithoutMoves minusSet:[NSSet setWithArray:change.deletions]];

            NSSet *addedOrModified = [insertionsWithoutMoves setByAddingObjectsFromArray:change.modifications];
            id<MUSSubscriptable> addedOrModifiedObjects = (id<MUSSubscriptable>)[results mus_objectsAtIndexes:addedOrModified.allObjects];

            [self processObjects:addedOrModifiedObjects forOperationType:type];
        }
    }];
}