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/addresses_pixels.py

@@ -0,0 +1,56 @@
+"""Model for the many-to-many relationship between `Address` and `Pixel` objects."""
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+
+from urban_meal_delivery.db import meta
+
+
+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.
+
+    Further info:
+        https://docs.sqlalchemy.org/en/stable/orm/basic_relationships.html#association-object  # noqa:E501
+    """
+
+    __tablename__ = 'addresses_pixels'
+
+    # Columns
+    _address_id = sa.Column('address_id', sa.Integer, primary_key=True)
+    _city_id = sa.Column('city_id', sa.SmallInteger, nullable=False)
+    _grid_id = sa.Column('grid_id', sa.SmallInteger, nullable=False)
+    _pixel_id = sa.Column('pixel_id', sa.Integer, primary_key=True)
+
+    # Constraints
+    __table_args__ = (
+        # An `Address` can only be on a `Grid` ...
+        sa.ForeignKeyConstraint(
+            ['address_id', 'city_id'],
+            ['addresses.id', 'addresses.city_id'],
+            onupdate='RESTRICT',
+            ondelete='RESTRICT',
+        ),
+        # ... if their `.city` attributes match.
+        sa.ForeignKeyConstraint(
+            ['grid_id', 'city_id'],
+            ['grids.id', 'grids.city_id'],
+            onupdate='RESTRICT',
+            ondelete='RESTRICT',
+        ),
+        # Each `Address` can only be on a `Grid` once.
+        sa.UniqueConstraint('address_id', 'grid_id'),
+        # An association must reference an existing `Grid`-`Pixel` pair.
+        sa.ForeignKeyConstraint(
+            ['pixel_id', 'grid_id'],
+            ['pixels.id', 'pixels.grid_id'],
+            onupdate='RESTRICT',
+            ondelete='RESTRICT',
+        ),
+    )
+
+    # Relationships
+    address = orm.relationship('Address', back_populates='pixels')
+    pixel = orm.relationship('Pixel', back_populates='addresses')