Make sure PreMixing customise uses new HF shower library
replicate standalone pythia8 behaviour for lhe energy-momentum treatment
Merge pull request #5812 from mdhildreth/PreMixing_Customise_update_for_HF

Make sure PreMixing PostLS1 customise uses new HF shower library
Merge pull request #5822 from bendavid/py8200final_71x

Backport of interface changes for final pythia 8.200 release (71x)
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X