Add aggregate_orders() function

- the function queries the database and aggregates the ad-hoc orders
  by pixel and time steps into a demand time series
- implement "heavy" integration tests for `aggregate_orders()`
- make `pandas` a package dependency
- streamline the `Config`

src/urban_meal_delivery/forecasts/__init__.py

@@ -0,0 +1,3 @@
+"""Demand forecasting utilities."""
+
+from urban_meal_delivery.forecasts import timify