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_cities.py

@@ -3,8 +3,9 @@
 
 import pytest
 
-from tests.db.utils import test_coordinates as consts
+from tests.db.utils import test_locations as consts
 from urban_meal_delivery import db
+from urban_meal_delivery.db import utils
 
 
 class TestSpecialMethods:
@@ -39,16 +40,22 @@ class TestConstraints:
 class TestProperties:
     """Test properties in `City`."""
 
-    def test_location_data(self, city, city_data):
-        """Test `City.location` property."""
-        result = city.location
+    def test_center(self, city, city_data):
+        """Test `City.center` property."""
+        result = city.center
 
-        assert isinstance(result, dict)
-        assert len(result) == 2
-        assert result['latitude'] == pytest.approx(city_data['_center_latitude'])
-        assert result['longitude'] == pytest.approx(city_data['_center_longitude'])
+        assert isinstance(result, utils.Location)
+        assert result.latitude == pytest.approx(city_data['_center_latitude'])
+        assert result.longitude == pytest.approx(city_data['_center_longitude'])
 
-    def test_viewport_data_overall(self, city):
+    def test_center_is_cached(self, city):
+        """Test `City.center` property."""
+        result1 = city.center
+        result2 = city.center
+
+        assert result1 is result2
+
+    def test_viewport_overall(self, city):
         """Test `City.viewport` property."""
         result = city.viewport
 
@@ -56,18 +63,24 @@ class TestProperties:
         assert len(result) == 2
 
     @pytest.mark.parametrize('corner', ['northeast', 'southwest'])
-    def test_viewport_data_corners(self, city, city_data, corner):
+    def test_viewport_corners(self, city, city_data, corner):
         """Test `City.viewport` property."""
         result = city.viewport[corner]
 
-        assert isinstance(result, dict)
-        assert len(result) == 2
-        assert result['latitude'] == pytest.approx(city_data[f'_{corner}_latitude'])
-        assert result['longitude'] == pytest.approx(city_data[f'_{corner}_longitude'])
+        assert isinstance(result, utils.Location)
+        assert result.latitude == pytest.approx(city_data[f'_{corner}_latitude'])
+        assert result.longitude == pytest.approx(city_data[f'_{corner}_longitude'])
 
-    def test_city_in_utm_coordinates(self, city):
-        """Test `City.as_origin` property."""
-        result = city.as_origin
+    def test_viewport_is_cached(self, city):
+        """Test `City.viewport` property."""
+        result1 = city.viewport
+        result2 = city.viewport
+
+        assert result1 is result2
+
+    def test_city_as_xy_origin(self, city):
+        """Test `City.as_xy_origin` property."""
+        result = city.as_xy_origin
 
         assert result.zone == consts.ZONE
         assert consts.MIN_EASTING < result.easting < consts.MAX_EASTING