Creation of header only package AtlasHepMC and migration of Generators to AtlasHepMC

Subset of !31532 (merged) Split into parts as suggested by @jchapman

This MR switches the Generator packages from hePMC to AtlasHepMC.

@akraszna , I've followed your instructions on Cmake, but so far haven't changed the namespace name. Should be considered later.

Andrii