[WIP] Measurement fixes

A few fixes and improvements in the cross section measurement workflow:
- [x] Updated configuration to work with the latest (AN-14-071_3rd_draft) set of histograms;
- [x] Apparently we were still changing both fit results input and the response matrices for ttbar generator systematics. Fixed that (now just changing the response matrix);
- [x] Implemented the proper calculation of the hadronisation systematic (difference between powheg_herwig and powheg_pythia). Powheg_herwig is not available for 7 TeV in 53X and not sure if it's in production. Alternatively, can use the old one from Fall11, 42X (/TT_AUET2_7TeV-powheg-herwig/Fall11-PU_S6_START42_V14B-v1/AODSIM);
- [x] Added the top pt reweighting MadGraph in the cross section plots, the truth histogram is from the unfolding workflow (not fit results like before). The corresponding systematic is also calculated correctly;
- [x] --ptreweight flag in the 02_unfold_and_measure.py script switches the central measurement to be unfolded with pt-reweighted MadGraph. Not used by default (let the ARC decide);
- [ ] WIP: properly implement PDF calculation (using a different response matrix for each PDF variation).