replace deep-copying of particles with shallow copying

Change the deep-copying of particles and flavourtags with the creation of a shallow selection.

Also, sits on top of !3586 (merged) which prefers Particle::Range over Particle::Container and which requires Analysis!1011 (closed) -- but those two can be applied independent of this MR, whereas this MR, in conjunction with LHCb!4193 (merged), will actually change Particle::Container to Particle::Range in a few places!

required for Moore!2592 (merged), requires in turn LHCb!4193 (merged)