Add experiment key to PLOTTING files

Created by: Zaharid

I think there is the general direction that things like #356 (closed) and #226 (closed), and also https://github.com/NNPDF/buildmaster/issues/42, should go in the direction of add various keys to the PLOTTING files.

Now that we have the bigexp in the fit, which is a hack. The PR to fix that is #226 (closed).

In the meantime we should just add an experiment key to the PLOTTING files. That would allow to produce tables with the chi² indexed by experiment.