Make the size of Pr::Seeding::Tracks container flexible

According to Composites structure defined in PhysEvent/Event/Particle_v2.h, adapt the Pr::Tracks containers with SOACollection to make the size flexible.

Firstly, take PrSeedTracks as an example and add test_prseedtracks.cpp to test the structure.

• Adapt the Long/Upstream/Velo tracks container with SOACollection will be in a following MR

Could you please have a look at it? @olupton We'd like to hear about your comments, thanks!

go with Rec!2167 (merged) @sstahl requires ref updates: Brunel!1067 (merged) Moore!611 (merged)