Support for multiple generation algorithm to simulate Beam-Gas + Beam-Beam collisions

Rebase of MR !811 (closed) - support for multiple Generation and consolidation of LbCRMC - and fix conflicts following MR 790 and 849

• setup of (multiple) collision systems through the Gauss Production option, e.g.
  Gauss().Production="BEAMGAS+BEAMBEAM"
  via multiple instantiations of the Generation algorithm

• CRMC/EPOS configuration from the Gauss Configurable through option
  Gauss().BeamBeam= ['CRMCEpos' ]
  and/or
  Gauss().BeamGas= ['CRMCEpos' ]
  (as was done for MadGraph)

• new Gauss options to declare fixed target configuration:
  FixedTargetParticle
  FixedTargetLuminosity
  FixedTargetXSection

• cleanup of CRMCProduction: kinematics set via BeamTool, output particles in Lab frame (BoostForEpos algorithm dropped)

• Pileup tool for Beam-gas collisions (BeamGasFixedLuminosity)

• Smear vertex smearing tool for gas in SMOG2 (TriangZSmearVertex)

A change to DecFiles is also required (corresponding merge request with same name)

Refer to !811 (closed) for additional information

cc: @ggrazian, @qiuchan, @sunj