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 (fixes #88)  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);
- [x] Fixed PDF calculation (using a different response matrix for each PDF variation). The PDFs are currently preliminary (x/y axes swapped and some are missing);
- [x] Commented out pt reweight systematic;
- [x] Tidied up tables, made compatible with the AN tdr
- [x] Fixed the bug in calculation of the rate-changing systematics (issue #110)