Use PLOTTING information to group by experiments

Created by: Zaharid

We currently typically enter data using the experiments key, but that is slowly going to be deprecated in favour of the data interface (#356 (closed)). The information of which dataset belongs to which experiment has been incorporated to the plotting files as per #412. We now would like that the various actions in validphys and particularly those in vp-comparefits group by the experiment defined in the metadata. They should probably warn if the set of experiments given as input contains something other than [BIGEXP].