Redo source forwarding

Allow source groups to be forwarded through another node. Originally this worked by simply adding input sources to the output streamer (in reconstruct), allowing all the input sources to be viewed by anyone connected to the reconstruct node. Due to the new group architecture this no longer works and a mechanism to allow source groups to be added to another group is required. This will involve more complex data copy strategy since normally buffer swaps are now used which prevents multiple observations of the same source.