Draft: Convert event number to long int

ATLAS have reported that the numbers of events to be generated in upcoming high-precision runs will overflow the capacity of int, which will cause problems since the HepMC event_number property is used as the proxy for the event numbers that will be merged together into ROOT-format event-record containers.

This branch changes the frontend, backend, and examples using event_number to use a long (or ROOT Long64_t) instead. It should be safely writeable from existing code, but reading from HepMC to ints in user code will need some care with very large run sizes.

I'm not sure about the implications for the ROOT persistence: is it smart enough to automatically handle the conversion between integer types, @averbyts ? (This also has to be addressed in the ATLAS TP converter code)