Add OrderHistory.first/last_order() methods

- get the `datetime` of the first or last order within a pixel
- unify some fixtures in `tests.forecasts.timify.conftest`

tests/forecasts/timify/test_make_time_series.py

@@ -11,51 +11,6 @@ import pytest
 
 from tests import config as test_config
 from urban_meal_delivery import config
-from urban_meal_delivery.forecasts import timify
-
-
-@pytest.fixture
-def good_pixel_id():
-    """A `pixel_id` that is on the `grid`."""
-    return 1
-
-
-@pytest.fixture
-def order_totals(good_pixel_id):
-    """A mock for `OrderHistory.totals`.
-
-    To be a bit more realistic, we sample two pixels on the `grid`.
-    """
-    pixel_ids = [good_pixel_id, good_pixel_id + 1]
-
-    gen = (
-        (pixel_id, start_at)
-        for pixel_id in pixel_ids
-        for start_at in pd.date_range(
-            test_config.START, test_config.END, freq=f'{test_config.LONG_TIME_STEP}T',
-        )
-        if config.SERVICE_START <= start_at.hour < config.SERVICE_END
-    )
-
-    # Re-index `data` filling in `0`s where there is no demand.
-    index = pd.MultiIndex.from_tuples(gen)
-    index.names = ['pixel_id', 'start_at']
-
-    df = pd.DataFrame(data={'n_orders': 0}, index=index)
-
-    # Sanity check: n_pixels * n_time_steps_per_day * n_weekdays * n_weeks.
-    assert len(df) == 2 * 12 * (7 * 2 + 1)
-
-    return df
-
-
-@pytest.fixture
-def order_history(order_totals, grid):
-    """An `OrderHistory` object that does not need the database."""
-    oh = timify.OrderHistory(grid=grid, time_step=test_config.LONG_TIME_STEP)
-    oh._data = order_totals
-
-    return oh
 
 
 @pytest.fixture