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/main.py

@@ -0,0 +1,37 @@
+"""The entry point for all CLI scripts in the project."""
+
+from typing import Any
+
+import click
+from click import core as cli_core
+
+import urban_meal_delivery
+
+
+def show_version(ctx: cli_core.Context, _param: Any, value: bool) -> None:
+    """Show the package's version."""
+    # If --version / -V is NOT passed in,
+    # continue with the command.
+    if not value or ctx.resilient_parsing:
+        return
+
+    # Mimic the colors of `poetry version`.
+    pkg_name = click.style(urban_meal_delivery.__pkg_name__, fg='green')  # noqa:WPS609
+    version = click.style(urban_meal_delivery.__version__, fg='blue')  # noqa:WPS609
+    # Include a warning for development versions.
+    warning = click.style(' (development)', fg='red') if '.dev' in version else ''
+    click.echo(f'{pkg_name}, version {version}{warning}')
+    ctx.exit()
+
+
+@click.group()
+@click.option(
+    '--version',
+    '-V',
+    is_flag=True,
+    callback=show_version,
+    is_eager=True,
+    expose_value=False,
+)
+def entry_point() -> None:
+    """The urban-meal-delivery research project."""