Add constructor for the DistanceMatrix class

- `DistanceMatrix.from_addresses()` takes a variable number of
  `Address` objects and creates distance matrix entries for them
- as a base measure, the air distance between two `Address`
  objects is calculated
- in addition, an integration with the Google Maps Directions API is
  implemented that provides a more realistic measure of the distance
  and duration a rider on a bicycle would need to travel between two
  `Address` objects
- add a `Location.lat_lng` convenience property that provides the
  `.latitude` and `.longitude` of an `Address` as a 2-`tuple`

tests/db/utils/test_locations.py

@@ -122,6 +122,15 @@ class TestProperties:
 
         assert result == pytest.approx(float(address.longitude))
 
+    def test_lat_lng(self, location, address):
+        """Test `Location.lat_lng` property."""
+        result = location.lat_lng
+
+        assert result == (
+            pytest.approx(float(address.latitude)),
+            pytest.approx(float(address.longitude)),
+        )
+
     def test_easting(self, location):
         """Test `Location.easting` property."""
         result = location.easting