Allow positive-negative rapidity symmetrisation into absolute rapidity for fastNLO tables

I found here: http://www.precision.hep.phy.cam.ac.uk/results/ttbar-fastnlo/ a fastNLO table which has signed rapidity in the binning, and which needs to be compared to an absolute rapidity cross section measurement. For this I find it useful to have a Symmetrise option for TermData. It would be nice to make this an option valid for all Reactions, but currently the numbers of bin check is done inside each Reaction code, which prevents from making it a global option for all Reactions.