Add missing public function to cengines

Author: Takishima

projectq/cengines/__init__.py

@@ -33,3 +33,5 @@
 from ._testengine import CompareEngine, DummyEngine
 from ._twodmapper import GridMapper
 from ._graphmapper import GraphMapper
+from ._gate_manager import (nearest_neighbours_cost_fun,
+                            look_ahead_parallelism_cost_fun)