- drop LHCb::v2::Event (as it won't "nest" properly)
- event model related classes go into LHCb::Event
- if there is a change, and the 'old' class needs to stick around,
then put the new class in a 'versioned' namespace, e.g.
namespace vNwhere N is one larger than the previous version number (as was eg. already done for v2::Track)
- adapt code to the above rule
- rename LHCb::v2::Tracks into LHCb::Event::v3::Tracks as v1 and v2 tracks were already known
- remove v2 from namespace of stuff that does not need to have a version