Pythia8 interface fix which is really working this time

When the bug in Pythia8 flat QCD weights was found, it was suggested to copy the Pythia8Interface from CMSSW_4_4_7 to fix it (https://github.com/cms-sw/cmssw/pull/3853). Even though it compiled nicely, the fix introduced some pythia8 initialization problems, probably due to name changes of functions in GeneratorInterface/Core, I think.

Therefore I made this new fix where I revert Pythia8Hadronizer.cc back to the former version and only updated the sections which deal with the weights. So this is a more minimal fix, as was actually suggested by Josh before. I have tested it with our generator fragments, and it is really working this time.