Use globals for the database connection

- remove the factory functions for creating engines and sessions
- define global engine, connection, and session objects to be used
  everywhere in the urban_meal_delivery package

research/clean_data.ipynb

@@ -103,8 +103,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "_engine = db.make_engine()\n",
-    "connection = _engine.connect()"
+    "connection = db.connection"
    ]
   },
   {