Add DistanceMatrix class

- the class stores the data of a distance matrix between all addresses
  + air distances
  + bicycle distances
- in addition, the "path" returned by the Google Directions API are
  also stored as a JSON serialized sequence of latitude-longitude pairs
- we assume a symmetric graph

src/urban_meal_delivery/db/addresses_pixels.py

@@ -10,7 +10,7 @@ class AddressPixelAssociation(meta.Base):
     """Association pattern between `Address` and `Pixel`.
 
     This approach is needed here mainly because it implicitly
-    updates the `_city_id` and `_grid_id` columns.
+    updates the `city_id` and `grid_id` columns.
 
     Further info:
         https://docs.sqlalchemy.org/en/stable/orm/basic_relationships.html#association-object  # noqa:E501