Add CLI script to gridify all cities

- reorganize `urban_meal_delivery.console` into a sub-package
- move `tests.db.conftest` fixtures into `tests.conftest`
  => some integration tests regarding CLI scripts need a database
- add `urban_meal_delivery.console.decorators.db_revision` decorator
  to ensure the database is at a certain state before a CLI script runs
- refactor the `urban_meal_delivery.db.grids.Grid.gridify()` constructor:
  - bug fix: even empty `Pixel`s end up in the database temporarily
    => create `Pixel` objects only if an `Address` is to be assigned
       to it
  - streamline code and docstring
  - add further test cases

src/urban_meal_delivery/console/gridify.py

@@ -0,0 +1,48 @@
+"""CLI script to create pixel grids."""
+
+import click
+
+from urban_meal_delivery import config
+from urban_meal_delivery import db
+from urban_meal_delivery.console import decorators
+
+
+@click.command()
+@decorators.db_revision('888e352d7526')
+def gridify() -> None:  # pragma: no cover  note:b1f68d24
+    """Create grids for all cities.
+
+    This command creates grids with pixels of various
+    side lengths (specified in `urban_meal_delivery.config`).
+
+    Pixels are only generated if they contain at least one
+    (pickup or delivery) address.
+
+    All data are persisted to the database.
+    """
+    cities = db.session.query(db.City).all()
+    click.echo(f'{len(cities)} cities retrieved from the database')
+
+    for city in cities:
+        click.echo(f'\nCreating grids for {city.name}')
+
+        for side_length in config.GRID_SIDE_LENGTHS:
+            click.echo(f'Creating grid with a side length of {side_length} meters')
+
+            grid = db.Grid.gridify(city=city, side_length=side_length)
+            db.session.add(grid)
+
+            click.echo(f' -> created {len(grid.pixels)} pixels')
+
+        # The number of assigned addresses is the same across different `side_length`s.
+        db.session.flush()  # necessary for the query to work
+        n_assigned = (
+            db.session.query(db.AddressPixelAssociation)
+            .filter(db.AddressPixelAssociation.grid_id == grid.id)
+            .count()
+        )
+        click.echo(
+            f'=> assigned {n_assigned} out of {len(city.addresses)} addresses in {city.name}',  # noqa:E501
+        )
+
+    db.session.commit()