added one more Sta API

Signed-off-by: Cho Moon <cmoon@precisioninno.com>

include/sta/Sta.hh

@@ -1325,6 +1325,7 @@ public:
   // Ensure that the timing graph has been built.
   Graph *ensureGraph();
   void ensureClkArrivals();
+  Levelize* levelize() const { return levelize_; }
 
   // Find all arc delays and vertex slews with delay calculator.
   virtual void findDelays();