add NuHepMC output format

[JostMigenda]

The NuHepMC output format (see arXiv:2310.13211) that’s been in the works for a while now looks like it’s almost ready. Notably, the new Hyper-K detector simulation expects to use this as the standard input format for all event generators; so sntools should add support for this output format.

See also the NuHepMC organisation on GitHub.