Draft: add madgraph+pythia8 to run2 mcmc mapping for GN2v01
@bdong reported some analyzers are using aMcAtNlo+Pythia8 MC samples. But we don't have MC-MC SF for this generator for GN2v01. In the current MR, aMcAtNlo+Pythia8 is mapping to the same generator as Pythia8EvtGen (assuming both are using the Pythia8 for showering and showering makes the dominant impact in btagging).
Could @dguest and @vvecchio please confirm if it is OK?
tagging @robouque