Should we support particle groups

[tkittel]

An advanced feature far down the road might be to support particle groups (aka "events"). The file writing interface would perhaps get something like:

   //If your particles should be grouped (into "events" typically),
   //mcpl_add_particle calls must come between calls to:
   void mcpl_begin_particle_group(mcpl_outfile_t);
   void mcpl_end_particle_group(mcpl_outfile_t);

And the file reading interface might get corresponding functions as well. If we want to keep constant-size particles, we might need a flag on all particles with internal group-id, and a special read method which can be used to loop over particles in a group. Challenge: support empty events (fake particle with negative group id? - a bit wasteful but could at least contain info about number of empty events represented). Better might be to add a single unsigned integer at the start of each event with nparticles, but would loose fast skip-ahead capability.

So far this was not requested by anyone and it seems like a lot of hassle. Nonetheless, it might come up and I am opening this issue so we can cleanup a TODO item that was previously listed in mcpl.c.

[tkittel]

Closing until someone actually wants this...