[ONOS-3222] Implement toString method for each Packet class

Change-Id: I17d72338d4202117d08b3dca9463be35a87a0c1e
diff --git a/utils/misc/src/main/java/org/onlab/packet/IGMPGroup.java b/utils/misc/src/main/java/org/onlab/packet/IGMPGroup.java
index 70ff556..8be75d4 100644
--- a/utils/misc/src/main/java/org/onlab/packet/IGMPGroup.java
+++ b/utils/misc/src/main/java/org/onlab/packet/IGMPGroup.java
@@ -95,4 +95,18 @@
      * @return The serialized message
      */
     public abstract byte[] serialize(ByteBuffer bb);
+
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        sb.append("[");
+        sb.append("auxInfo= ");
+        sb.append(auxInfo);
+        sb.append("gaddr= ");
+        sb.append(gaddr);
+        sb.append("sources= ");
+        sb.append(sources.toString());
+        sb.append("]");
+        return sb.toString();
+    }
 }