Add OrderHistory class

- the main purpose of this class is to manage querying the order totals
  from the database and slice various kinds of time series out of the
  data
- the class holds the former `aggregate_orders()` function as a method
- modularize the corresponding tests
- add `tests.config` with globals used when testing to provide a
  single source of truth for various settings

tests/forecasts/timify/test_order_history.py

@@ -0,0 +1,39 @@
+"""Test the basic functionalities in the `OrderHistory` class."""
+# pylint:disable=no-self-use
+
+import pytest
+
+from tests import config as test_config
+from urban_meal_delivery.forecasts import timify
+
+
+class TestSpecialMethods:
+    """Test the special methods in `OrderHistory`."""
+
+    @pytest.mark.parametrize('time_step', test_config.TIME_STEPS)
+    def test_instantiate(self, grid, time_step):
+        """Test `OrderHistory.__init__()`."""
+        oh = timify.OrderHistory(grid=grid, time_step=time_step)
+
+        assert oh is not None
+
+
+class TestProperties:
+    """Test the properties in `OrderHistory`."""
+
+    def test_totals_is_cached(self, grid, monkeypatch):
+        """Test `.totals` property.
+
+        The result of the `OrderHistory.aggregate_orders()` method call
+        is cached in the `OrderHistory.totals` property.
+        """
+        oh = timify.OrderHistory(grid=grid, time_step=test_config.LONG_TIME_STEP)
+
+        sentinel = object()
+        monkeypatch.setattr(oh, 'aggregate_orders', lambda: sentinel)
+
+        result1 = oh.totals
+        result2 = oh.totals
+
+        assert result1 is result2
+        assert result1 is sentinel