Add ORM models for the pixel grids

- add Grid, Pixel, and AddressPixelAssociation ORM models
- each Grid belongs to a City an is characterized by the side_length
  of all the square Pixels contained in it
- Pixels aggregate Addresses => many-to-many relationship (that is
  modeled with SQLAlchemy's Association Pattern to implement a couple
  of constraints)

tests/db/conftest.py

@@ -85,3 +85,5 @@ courier = fake_data.courier
 customer = fake_data.customer
 order = fake_data.order
 restaurant = fake_data.restaurant
+grid = fake_data.grid
+pixel = fake_data.pixel