WIP: Powheg+Pythia8 reweightings

Reweigthings for the Powheg+Pythia8 samples.

This MR is to follow the same procedure used by the TTHbbLeptonic analysis described in Table 1 (p.22) of HIGG-2017-03-022.pdf.

Useful links:

• TTbarModelling2016 providing an overview of the original studies
• Twiki pages by @mcasolin describing the HF classification variables used as input to the reweigthings HFclassificationForRun2, and some description of the reweigthings TtbbNLORw
• package maintained by @nasbah to apply the most recent reweigthings TTHbbTtbarCorrections
• example showing how these reweigthings are called within the TTHbbLeptonic code: TTbarReweightingTool.cxx
• list of samples used by TTHbbLeptonic: spreadsheet (refer to Table 1 for correspondence of systs)
  • 410501 : nominal
  • 41025{012} : Sherpa5F, plus b-filtered samples 41028{123}
  • 410525 : PS & hadronization, plus b-filtered samples 410528
  • 41051{12} : ISR/FSR, plus b-filtered samples 41051{78}
  • 41032{345} : tt+1b Sherpa4F
• example from @lvalery showing how a similar treatment was implemented for P+P6 in MainAnalysis.cxx within MultibjetsAnalysis: follow the variable dm_HFsyst

Tag for information @juste @orlando @lvalery @farooque