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`

tests/db/test_grids.py

@@ -19,7 +19,7 @@ class TestSpecialMethods:
         """`Grid` has a non-literal text representation."""
         result = repr(grid)
 
-        assert result == f'<Grid: {grid.pixel_area}>'
+        assert result == f'<Grid: {grid.pixel_area} sqr. km>'
 
 
 @pytest.mark.db