Rounding strategy for slot duration at JSON export

Implements #16 (closed)

AKModel/models.py

@@ -952,6 +952,7 @@ class AKSlot(models.Model):
                                    for owner in self.ak.owners.all()]),
                 "description": self.ak.description,
                 "reso": self.ak.reso,
+                "duration_in_hours": self.duration,
                 },
             }