Introduces ordering for services

services are now ordered by ascending odometer value instead of
their id

app/entities.py

@@ -77,7 +77,8 @@ class Vehicle(db.Model):
         order_by="asc(Pitstop.odometer)"
     )
     services = db.relationship(
-        'Service'
+        'Service',
+        order_by = "asc(Service.odometer)"
     )
     consumables = db.relationship(
         'Consumable',