Data preservation

[petitcactusorange]

From Roel :

• Backwards compatibility of the data files. Right now if a new object is
  stored in a ROOT file, the files becomes completely unreadable by old
  software versions although they would understand everything else fine. We
  should allow parts of the event to be "blindly" copied if they are not
  understood and everything we understand to be used. For the swimming this
  made things extremely painful.
• Data preservation. I think we should make proper specifications and
  versions of all event model classes such that they can always be
  understood later. This is currently sort of in the code, but not really on
  paper. It would be best if they were digital too, somehow.