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

setup.cfg

@@ -153,6 +153,9 @@ per-file-ignores =
     src/urban_meal_delivery/forecasts/methods/extrapolate_season.py:
         # The module is not too complex.
         WPS232,
+    src/urban_meal_delivery/forecasts/models/tactical/horizontal.py:
+        # The many noqa's are ok.
+        WPS403,
     src/urban_meal_delivery/forecasts/timify.py:
         # No SQL injection as the inputs come from a safe source.
         S608,