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
Edited by Andrii Verbytskyi