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

tests/db/test_addresses.py

@@ -6,6 +6,7 @@ import sqlalchemy as sqla
 from sqlalchemy import exc as sa_exc
 
 from urban_meal_delivery import db
+from urban_meal_delivery.db import utils
 
 
 class TestSpecialMethods:
@@ -123,6 +124,24 @@ class TestProperties:
 
         assert result is False
 
+    def test_location(self, address):
+        """Test `Address.location` property."""
+        latitude = float(address.latitude)
+        longitude = float(address.longitude)
+
+        result = address.location
+
+        assert isinstance(result, utils.Location)
+        assert result.latitude == pytest.approx(latitude)
+        assert result.longitude == pytest.approx(longitude)
+
+    def test_location_is_cached(self, address):
+        """Test `Address.location` property."""
+        result1 = address.location
+        result2 = address.location
+
+        assert result1 is result2
+
     def test_x_is_positive(self, address):
         """Test `Address.x` property."""
         result = address.x