CombineParticles can save subbranches

Added feature to CombineParticles: You can now use SelectionDescriptor to select which subbranch of the combined candidate you want to store. For example SelectionDescriptor='Beauty -> Beauty ^X' will select the tracks you added to you Beauty input particle. All cut machinery provided by CombineParticles works as usual when using this. If the SelectionDescriptor is not set, CombineParticles functionality remains exactly as before.