operator< for stateaction pairs

Former-commit-id: d69660a770

src/storage/StateActionPair.h

@@ -28,6 +28,9 @@ namespace storm {
                 return p1.stateActionPair != p2.stateActionPair;
             }
 
+            friend bool operator<(StateActionPair const& p1, StateActionPair const& p2) {
+                return p1.getState() < p2.getState() || (p1.getState() == p2.getState() && p1.getAction() < p2.getAction());
+            }
             
         };
     }