Run number and event number with truth samples

Moving the discussion to here from this comment.

When running over MCParticles (i.e. .xgen files), might be helpful to write event index for investigation related to multiple candidates (Run2 equivalent of EventInSequence). This is not a problem for reconstructed sample as both run and event numbers, retrived from ODIN, are written by default.

For MCParticles, one could consider:

• a brute-force way to write event index (EventInSequence) on C++ side in FunTuple at each event loop. Simpler and for uniformity could consider adding such a default variable when running over reconstructed sample too.
• or we could retrieve both run and event numbers, perhaps stored in MCHeader (as ODIN might only be created in Boole), that were used as seed for random number generation in Gauss. This option is in the spirit of functors, but I wonder if it is an overkill in this instance.

FYI: @pkoppenb , @erodrigu , @sstahl

This issue is linked from a comment in https://gitlab.cern.ch/lhcb/Analysis/-/blob/master/Phys/FunTuple/python/FunTuple/functorcollections.py#L60 which should be cleaned up once done.