New plugin module to convert GenParticles to HepMC format for Rivet analysis

New plugins module to do Rivet analysis with AODSIM data.
This module reproduces HepMC referring genParticles.

See 
  https://twiki.cern.ch/twiki/bin/view/CMS/RivetontoAODSIM
for overall information and links, 
and 
  https://twiki.cern.ch/twiki/bin/viewauth/CMS/GenParticles2HepMCConverter
for full documentation of this module.