Subscription & SubscriptionArray: add forcedUpdate() methods

Can be used to immediately get & use the data, as needed for the flight
tasks.

src/lib/FlightTasks/FlightTasks.hpp

@@ -151,6 +151,8 @@ public:
 			return -3;
 		}
 
+		_subscription_array.forcedUpdate(); // make sure data is available for all new subscriptions
+
 		_current_task_index = task_number;
 		return 0;
 	}

src/lib/FlightTasks/SubscriptionArray.cpp

@@ -76,3 +76,10 @@ void SubscriptionArray::update()
 		_subscriptions[i]->update();
 	}
 }
+
+void SubscriptionArray::forcedUpdate()
+{
+	for (int i = 0; i < _subscriptions_count; ++i) {
+		_subscriptions[i]->forcedUpdate();
+	}
+}

src/lib/FlightTasks/SubscriptionArray.hpp

@@ -60,10 +60,15 @@ public:
 	bool get(const struct orb_metadata *meta, uORB::Subscription<T> *&subscription, unsigned instance = 0);
 
 	/**
-	 * update all subscriptions
+	 * update all subscriptions (if new data is available)
 	 */
 	void update();
 
+	/**
+	 * update all subscriptions
+	 */
+	void forcedUpdate();
+
 private:
 	void cleanup();