ENH: Allows history to be dynamic and grow the container at runtime.

Previously, all specs had to be pre-allocated by using the 'add_history'
function. This is now no longer required and instead serves as a hint to
the HistoryContainer to pre-allocate the space for the given spec.

History can grow by increasing the length for a frequency, adding a
frequency, or adding a field. It can grow with any combination of
these.

HistoryContainer now is aware of the data_frequency of the algorithm,
and no longer uses the daily_at_midnight flag; instead, this is the
default behavior.

tests/test_batchtransform.py

@@ -122,8 +122,7 @@ class TestChangeOfSids(TestCase):
             for sid in self.sids[:i]:
                 self.assertIn(sid, df.columns)
 
-            last_elem = len(df) - 1
-            self.assertEqual(df[last_elem][last_elem], last_elem)
+            self.assertEqual(df.iloc[-1].iloc[-1], i)
 
 
 class TestBatchTransformMinutely(TestCase):