Anticipate how POS FK tables are parsed in the new commondata reader

Created by: Radonirinaunimi

I am not entirely sure yet how the positivity FK tables will be parsed in the new commondata reader. At one of the CMs, it was proposed to read the (x, Q^2) info from the FK tables directly therefore removing the need for a commondata file. However, it might still be better (and more flexible) to parse the positivity exactly in the same way as experimental data (as in the old commondata way). This is especially crucial for the polarised fit in which a positivity dataset is composed of compound FK tables with non-null operation.