Resolve "Specify internal input/output dependencies of an Operation"

Closes #74 (closed)

test/fixtures/port.py

@@ -10,3 +10,11 @@ def input_port():
 @pytest.fixture
 def output_port():
     return OutputPort(None, 0)
+
+@pytest.fixture
+def list_of_input_ports():
+    return [InputPort(None, i) for i in range(0, 3)]
+
+@pytest.fixture
+def list_of_output_ports():
+    return [OutputPort(None, i) for i in range(0, 3)]