Added a first example in GaudiExample featuring a tiny experiment code

The idea was to have a few, modern and functional, algorithms mimicing a full experiment cycle : generation, simulation, digitization, reconstruction and checker.

There is no I/O nor any concept of geometry for the moment. The event model is dead simple but still has Hits, Tracks, MCHits and MCTracks.

The associated test runs fully multithreaded using the AvalancheScheduler