Implements #134 source groups

Adds source groups to reconstruct using a new high precision timer abstraction. Gives an ability to precisely control latency and guarantee frame sync. Some work still required to better allow independence between receiving real frames and generate virtual viewpoints. Work also needed to remove callbacks for a clean shutdown.

There are many enhancements to network code efficiency also.

components/rgbd-sources/include/ftl/rgbd/group.hpp

@@ -54,6 +54,14 @@ class Group {
 	 */
 	void addSource(ftl::rgbd::Source *);
 
+	/**
+	 * Add another group to this one. All sources in the other group are made
+	 * available to this group in a synchronised way. There is additional
+	 * overhead in supporting this as additional data copies are required
+	 * internally for all the source frames.
+	 */
+	void addGroup(ftl::rgbd::Group *);
+
 	/**
 	 * Provide a function to be called once per frame with a valid frameset
 	 * at the specified latency. The function may not be called under certain