Modify ParticleCombination to make the type independent of the number of particles, remove DOCA{,Chi2} computation from combinations classes

• basically, change std::array to std::vector while keeping the 'outer' class backwards compatible
• change is required to be able to create a ParticleCombination from a (composite) LHCb::Particle
• remove computation of DOCA{,Chi2} from the combinations, and move it into relevant functors themselves
• add interfaces to combinations to allow the above point, namely: get, transform_reduce, pairwise_transform_reduce, pairwise_none_of
• double the number of doca-related functors: one set which uses a simple 'one state only' computation (prefixed with S, eg. SDOCA, with S for 'state'), and one set, the old and new default (which uses the IDistanceCalculator tool instead) -- but the latter only works for LHCb::Particle as the interface doesn't accept v2 classes...

Requires LHCb!3404 (merged), Moore!1301 (merged)