ENH: Adds last_available_{session, minute} args to DataPortal (#1528)

This allows optionally setting the last available dts in the DataPortal
explicitly. If these args aren't provided, we fall back to inferring
these from the underlying readers, which was the previous behavior.

zipline/data/dispatch_bar_reader.py

@@ -35,11 +35,21 @@ class AssetDispatchBarReader(with_metaclass(ABCMeta)):
     - readers : dict
         A dict mapping Asset type to the corresponding
         [Minute|Session]BarReader
+    - last_available_dt : pd.Timestamp or None, optional
+        If not provided, infers it by using the min of the
+        last_available_dt values of the underlying readers.
     """
-    def __init__(self, trading_calendar, asset_finder, readers):
+    def __init__(
+        self,
+        trading_calendar,
+        asset_finder,
+        readers,
+        last_available_dt=None,
+    ):
         self._trading_calendar = trading_calendar
         self._asset_finder = asset_finder
         self._readers = readers
+        self._last_available_dt = last_available_dt
 
         for t, r in iteritems(self._readers):
             assert trading_calendar == r.trading_calendar, \
@@ -72,7 +82,10 @@ class AssetDispatchBarReader(with_metaclass(ABCMeta)):
 
     @lazyval
     def last_available_dt(self):
-        return min(r.last_available_dt for r in self._readers.values())
+        if self._last_available_dt is not None:
+            return self._last_available_dt
+        else:
+            return min(r.last_available_dt for r in self._readers.values())
 
     @lazyval
     def first_trading_day(self):