Add Address.x and Address.y coordinates

- the Address.x and Address.y properties use the UTMCoordinate class
  behind the scenes
- x and y are simple coordinates in an x-y plane
- the (0, 0) origin is the southwest corner of Address.city.viewport

tests/db/test_addresses.py

@@ -122,3 +122,15 @@ class TestProperties:
         result = address.is_primary
 
         assert result is False
+
+    def test_x_is_positive(self, address):
+        """Test `Address.x` property."""
+        result = address.x
+
+        assert result > 0
+
+    def test_y_is_positive(self, address):
+        """Test `Address.y` property."""
+        result = address.y
+
+        assert result > 0