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/conftest.py

@@ -82,6 +82,17 @@ def good_pixel_id(pixel):
     return pixel.id  # `== 1`
 
 
+@pytest.fixture
+def predict_at() -> 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.fixture
 def order_totals(good_pixel_id):
     """A mock for `OrderHistory.totals`.