Rename DistanceMatrix into Path

- a `Path` is a better description for an instance of the model
- the `Location`s en route are renamed into `.waypoints`
- generic `assoc` is renamed into `path` in the test suite

src/urban_meal_delivery/db/__init__.py

@@ -1,7 +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_addresses import Path
 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