Migrate everything to AtlasHepMC

In this MR:

• all headers "HepMC/" are replaced with "AtlasHepMC/"

• removed the find( HepMC )

• replaced HEPMC_LIBRARIES with AtlasHepMCLib

• removed HEPMC_INCLUDE_DIR

• put Generators/AtlasHepMC to the list of directory dependencies for all updated packages

• all forward declarations of HepMC classes replaced with inclusions of the corresponding forward headers from AtlasHepMC.

• added one forward header to AtlasHepMC. IO_GenEvent_fwd.h.

Tagging @jchapman @ewelina @akraszna

Andrii