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/__init__.py

@@ -1,6 +1,7 @@
 """Provide the ORM models and a connection to the database."""
 
 from urban_meal_delivery.db.addresses import Address
+from urban_meal_delivery.db.addresses_addresses import DistanceMatrix
 from urban_meal_delivery.db.addresses_pixels import AddressPixelAssociation
 from urban_meal_delivery.db.cities import City
 from urban_meal_delivery.db.connection import connection