HepMC3 interface and upate of HepMC2 interface

In this MR:

• Updates of HepMC2 interface
• Implementation of HepMC3 interface

A more detailed description by file:

• include/Makefile.am -- added interface files
• include/HepMCSteering.inc include/Pythia6ToHepMC.inc include/Pythia6ToHepMC2.inc include/Pythia6ToHepMC3.inc interface files
• manual/rapgap32.tex update docs for new options
• src/rapgap/Pythia6ToHepMC.cc src/rapgap/Pythia6ToHepMC2.cc src/rapgap/Pythia6ToHepMC3.cc -- interface files
• src/rapgap/rgmain-hepmc.F -- added the handling of new interface
• src/Makefile.am --- added the new interface. Applied more fine grained flags, e.g. -I$(HEPMC2)/include is now added only to rapgap_hepmc and only as CXXFLAG. added -flat_namespace flag for Darwin builds, see 1)
• .gitlab-ci.yml added HepMC3 builds in CI
• Added HepMC3. Added library soversions, used more fine-grained flags. Added an option to handle RPATH on Mac in non-Linux way.
• configure.ac Added HepMC3. Added /lib64 to the search path of LHPADF libs. Added platform detection macro. Added AC_CONFIG_HEADERS([config.h]) to prevent huge amount of spam flags from being added to the command line. Removed -fPIC from MYFFLAGS, as that is an option that should be taken care by libtool.

1. libtool is used to link the libraries in RAPGAP. But the libtool https://www.gnu.org/software/libtool/ is not maintained well and was updated last time in ~ 2015. Some patches are needed to support the newest OSes like BigSrr Meanwhile, a "manual" addition of flags is a workaround. See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97865 https://github.com/Homebrew/homebrew-core/blob/master/Formula/libtool.rb

Best regards,

Andrii