Algorithm to handle a selection of particles as output of a functor

Algorithm that retrieves a selection of particles after the application of the functor. Several functors are now retrieving selection of particles different from the original one, it could be useful to do operations directly using these selections instead of the original one.

An example is presented here, where we need to persist extra tracks just for given candidates coming of the signal instead of all candidates (see application here).

The advantage will be in saving extra particles just for the limited number of candidates coming from signal.

cc: @cmarinbe, @sstahl functionality discussed during the hackathon

Note: For the moment the algorithm is limmited to the case when the output of the functor is a LHCb::Particle::ConstVector (decay tree functors output)