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/pixels.py

@@ -0,0 +1,59 @@
+"""Provide the ORM's `Pixel` model."""
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+
+from urban_meal_delivery.db import meta
+
+
+class Pixel(meta.Base):
+    """A pixel in a `Grid`.
+
+    Square pixels aggregate `Address` objects within a `City`.
+    Every `Address` belongs to exactly one `Pixel` in a `Grid`.
+
+    Every `Pixel` has a unique "coordinate" within the `Grid`.
+    """
+
+    __tablename__ = 'pixels'
+
+    # Columns
+    id = sa.Column(sa.Integer, primary_key=True, autoincrement=True)  # noqa:WPS125
+    _grid_id = sa.Column('grid_id', sa.SmallInteger, nullable=False, index=True)
+    n_x = sa.Column(sa.SmallInteger, nullable=False, index=True)
+    n_y = sa.Column(sa.SmallInteger, nullable=False, index=True)
+
+    # Constraints
+    __table_args__ = (
+        sa.ForeignKeyConstraint(
+            ['grid_id'], ['grids.id'], onupdate='RESTRICT', ondelete='RESTRICT',
+        ),
+        sa.CheckConstraint('0 <= n_x', name='n_x_is_positive'),
+        sa.CheckConstraint('0 <= n_y', name='n_y_is_positive'),
+        # Needed by a `ForeignKeyConstraint` in `AddressPixelAssociation`.
+        sa.UniqueConstraint('id', 'grid_id'),
+        # Each coordinate within the same `grid` is used at most once.
+        sa.UniqueConstraint('grid_id', 'n_x', 'n_y'),
+    )
+
+    # Relationships
+    grid = orm.relationship('Grid', back_populates='pixels')
+    addresses = orm.relationship('AddressPixelAssociation', back_populates='pixel')
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<{cls}: ({x}, {y})>'.format(
+            cls=self.__class__.__name__, x=self.n_x, y=self.n_y,
+        )
+
+    # Convenience properties
+
+    @property
+    def side_length(self) -> int:
+        """The length of one side of a pixel in meters."""
+        return self.grid.side_length
+
+    @property
+    def area(self) -> float:
+        """The area of a pixel in square kilometers."""
+        return self.grid.pixel_area