fix #68 calculation of diff with previous value not correct if readin…

…gs less than 5 minutes apart

Calculation of diff was taking the slope and then multiplied with 5 minutes. This seems not correct (strange nobody every noticed ?)

Probably because originally this was written for Dexcom transmitters, which give a reading every 5 minutes.

Although there too the diff is not correct if time difference > 5 minutes (10 minutes, 15, ...)

This changes will give another diff value in home screen, speak reading, watch, notification, text in alert.

Triggering of alerts is not impacted.

xdrip/Core Data/classes/BgReading+CoreDataClass.swift

@@ -169,7 +169,9 @@ public class BgReading: NSManagedObject {
             return "???";
         }
 
-        let value = currentSlope(previousBgReading: previousBgReading) * 5 * 60 * 1000;
+        // delta value recalculated aligned with time difference between previous and this reading
+        let value = currentSlope(previousBgReading: previousBgReading) * timeStamp.timeIntervalSince(previousBgReading.timeStamp) * 1000;
+
         if(abs(value) > 100){
             // a delta > 100 will not happen with real BG values -> problematic sensor data
             return "ERR";