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.

tests/test_data_portal.py

@@ -23,13 +23,14 @@ from zipline.assets import Equity
 from zipline.testing.fixtures import (
     ZiplineTestCase,
     WithTradingSessions,
-    WithDataPortal
+    WithDataPortal,
+    alias,
 )
 
 
-class TestDataPortal(WithDataPortal,
-                     WithTradingSessions,
-                     ZiplineTestCase):
+class DataPortalTestBase(WithDataPortal,
+                         WithTradingSessions,
+                         ZiplineTestCase):
 
     ASSET_FINDER_EQUITY_SIDS = (1,)
     START_DATE = pd.Timestamp('2016-08-01')
@@ -374,3 +375,13 @@ class TestDataPortal(WithDataPortal,
         self.assertEqual(minutes[4], result,
                          "Asset 10000 had a trade on fourth minute, so should "
                          "return that as the last trade on the fifth.")
+
+
+class TestDataPortal(DataPortalTestBase):
+    DATA_PORTAL_LAST_AVAILABLE_SESSION = None
+    DATA_PORTAL_LAST_AVAILABLE_MINUTE = None
+
+
+class TestDataPortalExplicitLastAvailable(DataPortalTestBase):
+    DATA_PORTAL_LAST_AVAILABLE_SESSION = alias('START_DATE')
+    DATA_PORTAL_LAST_AVAILABLE_MINUTE = alias('END_DATE')