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/grids.py

@@ -0,0 +1,48 @@
+"""Provide the ORM's `Grid` model."""
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+
+from urban_meal_delivery.db import meta
+
+
+class Grid(meta.Base):
+    """A grid of `Pixel`s to partition a `City`.
+
+    A grid is characterized by the uniform size of the `Pixel`s it contains.
+    That is configures via the `Grid.side_length` attribute.
+    """
+
+    __tablename__ = 'grids'
+
+    # Columns
+    id = sa.Column(  # noqa:WPS125
+        sa.SmallInteger, primary_key=True, autoincrement=True,
+    )
+    _city_id = sa.Column('city_id', sa.SmallInteger, nullable=False)
+    side_length = sa.Column(sa.SmallInteger, nullable=False, unique=True)
+
+    # Constraints
+    __table_args__ = (
+        sa.ForeignKeyConstraint(
+            ['city_id'], ['cities.id'], onupdate='RESTRICT', ondelete='RESTRICT',
+        ),
+        # Needed by a `ForeignKeyConstraint` in `address_pixel_association`.
+        sa.UniqueConstraint('id', 'city_id'),
+    )
+
+    # Relationships
+    city = orm.relationship('City', back_populates='grids')
+    pixels = orm.relationship('Pixel', back_populates='grid')
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<{cls}: {area}>'.format(
+            cls=self.__class__.__name__, area=self.pixel_area,
+        )
+
+    # Convenience properties
+    @property
+    def pixel_area(self) -> float:
+        """The area of a `Pixel` on the grid in square kilometers."""
+        return (self.side_length ** 2) / 1_000_000  # noqa:WPS432