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/TopologyManager.java b/src/main/java/net/onrc/onos/core/topology/TopologyManager.java
index 31933e4..62228f0 100644
--- a/src/main/java/net/onrc/onos/core/topology/TopologyManager.java
+++ b/src/main/java/net/onrc/onos/core/topology/TopologyManager.java
@@ -130,11 +130,11 @@
     }
 
     /**
-     * Get the Topology.
+     * Get the MutableTopology.
      *
-     * @return the Topology.
+     * @return the MutableTopology.
      */
-    Topology getTopology() {
+    MutableTopology getTopology() {
         return topology;
     }