Add Lua functions to print cluster id (closes #704)

include/openspace/engine/wrapper/sgctwindowwrapper.h

@@ -65,6 +65,7 @@ public:
     bool hasGuiWindow() const override;
     bool isGuiWindow() const override;
     bool isMaster() const override;
+    int clusterId() const override;
     bool isUsingSwapGroups() const override;
     bool isSwapGroupMaster() const override;
 

include/openspace/engine/wrapper/windowwrapper.h

@@ -207,6 +207,13 @@ public:
      */
     virtual bool isMaster() const;
 
+    /**
+     * Returns the unique identifier of this OpenSpace instance within the current cluster
+     * configuration. If this instance is not part of a cluster, this value is always 0.
+     * \return The cluster identifier of this OpenSpace instance
+     */
+    virtual int clusterId() const;
+
     /**
     * Returns <code>true</code> if the current rendering window is using swap groups.
     */