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_term.py

@@ -6,6 +6,7 @@ from itertools import product
 from unittest import TestCase
 
 from toolz import assoc
+import pandas as pd
 
 from zipline.assets import Asset
 from zipline.errors import (
@@ -24,7 +25,7 @@ from zipline.pipeline import (
     CustomFactor,
     Factor,
     Filter,
-    TermGraph,
+    ExecutionPlan,
 )
 from zipline.pipeline.data import Column, DataSet
 from zipline.pipeline.data.testing import TestingDataSet
@@ -33,6 +34,7 @@ from zipline.pipeline.factors import RecarrayField
 from zipline.pipeline.sentinels import NotSpecified
 from zipline.pipeline.term import AssetExists, Slice
 from zipline.testing import parameter_space
+from zipline.testing.fixtures import WithTradingSessions, ZiplineTestCase
 from zipline.testing.predicates import (
     assert_equal,
     assert_raises,
@@ -152,7 +154,14 @@ def to_dict(l):
     return dict(zip(map(str, range(len(l))), l))
 
 
-class DependencyResolutionTestCase(TestCase):
+class DependencyResolutionTestCase(WithTradingSessions, ZiplineTestCase):
+
+    TRADING_CALENDAR_STRS = ('NYSE',)
+    START_DATE = pd.Timestamp('2014-01-02', tz='UTC')
+    END_DATE = pd.Timestamp('2014-12-31', tz='UTC')
+
+    execution_plan_start = pd.Timestamp('2014-06-01', tz='UTC')
+    execution_plan_end = pd.Timestamp('2014-06-30', tz='UTC')
 
     def check_dependency_order(self, ordered_terms):
         seen = set()
@@ -163,6 +172,14 @@ class DependencyResolutionTestCase(TestCase):
 
             seen.add(term)
 
+    def make_execution_plan(self, terms):
+        return ExecutionPlan(
+            terms,
+            self.nyse_sessions,
+            self.execution_plan_start,
+            self.execution_plan_end,
+        )
+
     def test_single_factor(self):
         """
         Test dependency resolution for a single factor.
@@ -182,7 +199,7 @@ class DependencyResolutionTestCase(TestCase):
             self.assertEqual(graph.node[SomeDataSet.bar]['extra_rows'], 4)
 
         for foobar in gen_equivalent_factors():
-            check_output(TermGraph(to_dict([foobar])))
+            check_output(self.make_execution_plan(to_dict([foobar])))
 
     def test_single_factor_instance_args(self):
         """
@@ -190,7 +207,9 @@ class DependencyResolutionTestCase(TestCase):
         the constructor.
         """
         bar, buzz = SomeDataSet.bar, SomeDataSet.buzz
-        graph = TermGraph(to_dict([SomeFactor([bar, buzz], window_length=5)]))
+
+        factor = SomeFactor([bar, buzz], window_length=5)
+        graph = self.make_execution_plan(to_dict([factor]))
 
         resolution_order = list(graph.ordered())
 
@@ -214,7 +233,7 @@ class DependencyResolutionTestCase(TestCase):
         f1 = SomeFactor([SomeDataSet.foo, SomeDataSet.bar])
         f2 = SomeOtherFactor([SomeDataSet.bar, SomeDataSet.buzz])
 
-        graph = TermGraph(to_dict([f1, f2]))
+        graph = self.make_execution_plan(to_dict([f1, f2]))
         resolution_order = list(graph.ordered())
 
         # bar should only appear once.