Add OrderHistory.make_horizontal_time_series()

- the method slices out a horizontal time series from the data within
  an `OrderHistory` object

tests/config.py

@@ -1,10 +1,28 @@
 """Globals used when testing."""
 
+import datetime
+
+from urban_meal_delivery import config
+
 
 # The day on which most test cases take place.
 YEAR, MONTH, DAY = 2016, 7, 1
 
+# `START` and `END` constitute a 15-day time span.
+# That implies a maximum `train_horizon` of `2` as that needs full 7-day weeks.
+START = datetime.datetime(YEAR, MONTH, DAY, config.SERVICE_START, 0)
+_end_day = (START + datetime.timedelta(weeks=2)).date()
+END = datetime.datetime(
+    _end_day.year, _end_day.month, _end_day.day, config.SERVICE_END, 0,
+)
+
 # Default time steps, for example, for `OrderHistory` objects.
 LONG_TIME_STEP = 60
 SHORT_TIME_STEP = 30
 TIME_STEPS = (SHORT_TIME_STEP, LONG_TIME_STEP)
+
+# Default training horizons, for example, for
+# `OrderHistory.make_horizontal_time_series()`.
+LONG_TRAIN_HORIZON = 2
+SHORT_TRAIN_HORIZON = 1
+TRAIN_HORIZONS = (SHORT_TRAIN_HORIZON, LONG_TRAIN_HORIZON)