MAINT: Remove notion of "atomic" pipeline terms.

Replace it by distinguishing between "Loadable" and "Computable".

This is useful because it's now  possible to write computable terms that
don't require  any inputs  (e.g. an `Always`  filter or  an `Everything`
classifier).

tests/pipeline/test_term.py

@@ -7,7 +7,7 @@ from unittest import TestCase
 
 from zipline.errors import (
     DTypeNotSpecified,
-    InputTermNotAtomic,
+    WindowedInputToWindowedTerm,
     NotDType,
     TermInputsNotSpecified,
     UnsupportedDType,
@@ -157,7 +157,7 @@ class DependencyResolutionTestCase(TestCase):
         self.assertEqual(graph.extra_rows[bar], 4)
         self.assertEqual(graph.extra_rows[buzz], 4)
 
-    def test_reuse_atomic_terms(self):
+    def test_reuse_loadable_terms(self):
         """
         Test that raw inputs only show up in the dependency graph once.
         """
@@ -174,7 +174,7 @@ class DependencyResolutionTestCase(TestCase):
 
     def test_disallow_recursive_lookback(self):
 
-        with self.assertRaises(InputTermNotAtomic):
+        with self.assertRaises(WindowedInputToWindowedTerm):
             SomeFactor(inputs=[SomeFactor(), SomeDataSet.foo])