HepMC option to stdout

By default EPOS4 is not able to print HepMC data to stdout, so an option, set with "ihepmc 2", has been introduced. This is useful to make the current O2 generation mechanism work with this generator. Moreover a common seed flag has been added to the epos script, as well as a -hepstd flag that must be activated to print most messages in a .mtr file, while keeping the HepMC output on stdout. This last option is fundamental, otherwise the output of warning messages gets interleaved with the HepMC one.