MAINT: Refactor in prep for downsampled terms.

- Split out extra_rows handling into an `ExecutionPlan` subclass.
  `ExecutionPlan` now requires the dates and calendar against which a
  set of terms will be computed, and now defers to a term's
  `compute_extra_rows` method when deciding how many extra rows are
  required to compute for that term. This will allow downsampled terms
  to request enough extra rows to guarantee that we can maintain consistent
  calculation dates.

  As a consequence of the above, `TermGraph` now only deals with logical
  dependencies, not with metadata surrounding extra row calculations.
  This means that TermGraph can be used to generate dependency
  visualizations in interactive contexts where we don't yet have a
  calendar or start/end dates.

- Refactored test_{filter,factor,classifier} to use check_terms instead
  of run_graph.  This makes it easier to make changes to TermGraph,
  since the testing interface is now to simply provide a dict of terms.

- Refactored BasePipelineTestCase to use fixtures to create an asset
  finder.  This fixes a potential leak of the test's asset db, which was
  not being explicitly cleaned up.

- Refactored test_technical to use BasePipelineTestCase.

- Added a new special term, `InputDates()`, which can be used to request
  date labels for inputs.  Like `AssetExists`, `InputDates` is provided
  in the initial workspace by default.

- Added a default (failing) `_compute` method to `AssetExists` which
  provides a more useful error than AttributeError.

tests/pipeline/test_engine.py

@@ -40,6 +40,7 @@ from six import iteritems, itervalues
 from toolz import merge
 
 from zipline.assets.synthetic import make_rotating_equity_info
+from zipline.errors import NoFurtherDataError
 from zipline.lib.adjustment import MULTIPLY
 from zipline.lib.labelarray import LabelArray
 from zipline.pipeline import CustomFactor, Pipeline
@@ -65,6 +66,7 @@ from zipline.pipeline.loaders.synthetic import (
     expected_bar_values_2d,
 )
 from zipline.pipeline.sentinels import NotSpecified
+from zipline.pipeline.term import InputDates
 from zipline.testing import (
     AssetID,
     AssetIDPlusDay,
@@ -81,7 +83,7 @@ from zipline.testing.fixtures import (
     ZiplineTestCase,
 )
 from zipline.utils.memoize import lazyval
-from zipline.utils.numpy_utils import bool_dtype
+from zipline.utils.numpy_utils import bool_dtype, datetime64ns_dtype
 
 
 class RollingSumDifference(CustomFactor):
@@ -229,6 +231,31 @@ class ConstantInputTestCase(WithTradingEnvironment, ZiplineTestCase):
         with self.assertRaises(NoFurtherDataError) as e:
             engine.run_pipeline(p, self.dates[8], self.dates[8])
 
+    def test_input_dates_provided_by_default(self):
+        loader = self.loader
+        engine = SimplePipelineEngine(
+            lambda column: loader, self.dates, self.asset_finder,
+        )
+
+        class TestFactor(CustomFactor):
+            inputs = [InputDates(), USEquityPricing.close]
+            window_length = 10
+            dtype = datetime64ns_dtype
+
+            def compute(self, today, assets, out, dates, closes):
+                first, last = dates[[0, -1], 0]
+                assert last == today.asm8
+                assert len(dates) == len(closes) == self.window_length
+                out[:] = first
+
+        p = Pipeline(columns={'t': TestFactor()})
+        results = engine.run_pipeline(p, self.dates[9], self.dates[10])
+
+        # All results are the same, so just grab one column.
+        column = results.unstack().iloc[:, 0].values
+        check_arrays(column, self.dates[:2].values)
+
+
     def test_same_day_pipeline(self):
         loader = self.loader
         engine = SimplePipelineEngine(