Change GenParticles path name in pythia.py to MCParticles to match what ddsim expects with edm4hep input

[davidlange6]

Following gen then fullsim tutorials, this PR fixes the following error for using pythia events as input to ddsim

k4run pythia_org.py -n 50 --out.filename p8_mumu_500.e4h_orig.root \
  --Pythia8.PythiaInterface.pythiacard p8_ee_Zmumu_ecm91.cmd

and then

ddsim --compactFile $K4GEO/FCCee/CLD/compact/CLD_o2_v07/CLD_o2_v07.xml\
    --outputFile test.root\
    --skipNEvents 0\
    --numberOfEvents 2000\
    -I p8_mumu_500.e4h_orig.root\
    --steering cld_steer.py

currently results in

EDM4hepFileReader INFO  Created file reader. Try to open input ../gen/p8_mumu_500.e4h_orig.root
Geant4Output2EDM4hep INFO  Opened ee_Zbb_ecm91_LbLGamma.signalLbLGamma_ddsim_out_orig.root for output
SignalHandler    INFO  ++ Re-apply signal handlers
GenerationInit   INFO  +++ Initializing event 1. Within run:0 event 1.
Geant4Kernel     FATAL +++ Exception while simulating:Cannot retrieve collection MCParticles of type edm4hep::MCParticleCollection
DDSim            ERROR Simulation failed!

BEGINRELEASENOTES

• change GenParticles path name in pythia.py to MCParticles to match what ddsim expects with edm4hep input

ENDRELEASENOTES

[jmcarcell]

The problem I see with this is that basically everywhere in this repository, the default is GenParticles. I would say a better change would be to use MCParticles everywhere but I don't use this repository. Maybe @kjvbrt has an opinion.