Add OrderHistory.avg_daily_demand()

- the method calculates the number of daily `Order`s in a `Pixel`
  withing the `train_horizon` preceding the `predict_day`

tests/forecasts/test_models.py

@@ -1,6 +1,5 @@
 """Tests for the `urban_meal_delivery.forecasts.models` sub-package."""
 
-import datetime as dt
 
 import pandas as pd
 import pytest
@@ -59,16 +58,6 @@ class TestGenericForecastingModelProperties:
 
         self.unique_model_names.add(model.name)
 
-    @pytest.fixture
-    def predict_at(self) -> dt.datetime:
-        """`NOON` on the day to be predicted."""
-        return dt.datetime(
-            test_config.END.year,
-            test_config.END.month,
-            test_config.END.day,
-            test_config.NOON,
-        )
-
     @pytest.mark.r
     def test_make_prediction_structure(
         self, model_cls, order_history, pixel, predict_at,