Vector cost proposed to TST on 2016-07-13

First part implemented: weight interface introduced and integrated, default weight implementation added.

Change-Id: Ia46f1b44139069aa171a3c13faf168351bd7cc56
diff --git a/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java b/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java
index 1e8d3d6..6436ad4 100644
--- a/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java
+++ b/utils/misc/src/main/java/org/onlab/graph/DisjointPathPair.java
@@ -73,8 +73,8 @@
     }
 
     @Override
-    public double cost() {
-        return hasBackup() ? primary.cost() + secondary.cost() : primary.cost();
+    public Weight cost() {
+        return hasBackup() ? primary.cost().merge(secondary.cost()) : primary.cost();
     }
 
     @Override