Add HorizontalSMAModel

- the model applies a simple moving average on horizontal time series
- refactor `db.Forecast.from_dataframe()` to correctly convert
  `float('NaN')` values into `None`; otherwise, SQLAlchemy complains

tests/forecasts/test_models.py

@@ -11,6 +11,7 @@ from urban_meal_delivery.forecasts import models
 
 MODELS = (
     models.HorizontalETSModel,
+    models.HorizontalSMAModel,
     models.RealtimeARIMAModel,
     models.VerticalARIMAModel,
 )