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)

src/urban_meal_delivery/db/cities.py

@@ -45,6 +45,7 @@ class City(meta.Base):
 
     # Relationships
     addresses = orm.relationship('Address', back_populates='city')
+    grids = orm.relationship('Grid', back_populates='city')
 
     def __init__(self, *args: Any, **kwargs: Any) -> None:
         """Create a new city."""