Improvement of JEC function and implementation of 2017 corrections

Improvement of JEC function and implementation of 2017 corrections, I added a switch to select the year and the corresponding corrections, at the moment the uncertainties are calculated in case III, i.e. one single variations (as described here). the possibility of changing between case III and case II could be a further improvement.

I also added two functions to fetch correction txt files from EOS, the two are based on pre-existing functions but I couldn't test them due to a copy error (which is also showing on my side for other EOS copy like PU corrections files)