Fixing topology related interface/class tree

- (Mutable)Topology no longer implements ImmutableTopology

    --- BaseTopology (was ImmutableTopology)
     |
     +-- ImmutableTopology (new)
     |
     +-- MutableTopology (was Topology)
       |
       +-- RefreshableTopology
       |
       +-- TopologyInternal
            This interface is placed in wrong place. To be fixed later.

Change-Id: Ifa9defcc7c0fc22bba19977fe4ea57eaf792275e
diff --git a/src/main/java/net/onrc/onos/core/topology/TopologyModule.java b/src/main/java/net/onrc/onos/core/topology/TopologyModule.java
index 07ba2cf..7913fc5 100644
--- a/src/main/java/net/onrc/onos/core/topology/TopologyModule.java
+++ b/src/main/java/net/onrc/onos/core/topology/TopologyModule.java
@@ -67,7 +67,7 @@
     }
 
     @Override
-    public Topology getTopology() {
+    public MutableTopology getTopology() {
         return topologyManager.getTopology();
     }