Adding a functionality to treat `long long int` event numbers

In this MR:

• Adding a functionality to treat long long int event numbers with functions set_ll_event_number and get_ll_event_number. Both functions are available for HepMC2 and HepMC3 versions, but in the HepMC2 version, the numbers outside of the int range are rejected. In HepMC3, if the required attribute is missing, the event_number() is used instead. Practically this will allow dealing with long long int event numbers, but to actually do that one should update Athena codes. And know exactly where to do that.

• Just in case, it could be a good idea to keep the _ll_ part in these functions so if there will be an idea to promote the event_number in Athena to something else, e.g. uint64_t, one would not confuse the functions.

• The MR does not update any currently used codes, but just adds a new functionality that are claimed to be required in the future.

Best regards,

Andrii

@jchapman @cgutscho