Integrate the new Location class

- the old `UTMCoordinate` class becomes the new `Location` class
- its main purpose is to represent locations in both lat-long
  coordinates as well as in the UTM system
- remove `Address.__init__()` and `City.__init__()` methods as they
  are not executed for entries retrieved from the database
- simplfiy the `Location.__init__()` => remove `relative_to` argument

src/urban_meal_delivery/db/utils/__init__.py

@@ -1,3 +1,3 @@
 """Utilities used by the ORM models."""
 
-from urban_meal_delivery.db.utils.coordinates import UTMCoordinate  # noqa:F401
+from urban_meal_delivery.db.utils.locations import Location