refactor the ETA estimation

aman/sys/aco/RunwayManager.py

@@ -50,13 +50,11 @@ class RunwayManager:
             arrivalTime = node.ArrivalCandidates[runway].InitialArrivalTime
 
         if None == constrainedETA:
-            return max(arrivalTime, earliestArrivalTime), timedelta(seconds = 0)
+            eta = max(arrivalTime, earliestArrivalTime)
         else:
             eta = max(constrainedETA, earliestArrivalTime)
-            if eta < arrivalTime:
-                return arrivalTime, arrivalTime - eta
-            else:
-                return eta, timedelta(seconds = 0)
+
+        return eta, eta - arrivalTime
 
     def selectShallShouldMayArrivalRunway(self, node : Node, runways, useETA : bool, earliestArrivalTime : datetime):
         candidate = None