MCRelationship dictionary access for ROOT users

[AndrewEdmonds11]

As described in a comment of #181, ROOT users will also need the dictionary for MCRelationship in their library path. This is in Offline and not TrkAna and so they will need to have access to a build of Offline.

Ideally we would have no need for Offline when using the ntuple and so another way around this would be good. A couple of ideas for someone to try:

• add MCRelationship to the classes_def.xml of TrkAna
  • will this cause issues with people who build Offline and TrkAna together?
• do something similar to SurfaceId where we store the underlying ints
  • we would then lose all the helper functions...

If neither of these are feasible, then we may need to discuss a different mechanism for providing dictionaries to analyzers (e.g. a script that puts the relevant ones into the library path regardless of which repo they are in).