Allow for a 4th optional kinematic variable.

Created by: giacomomagni

To properly descrive some dijets measurements I'd like to allow for a 4th kinematic variable. This would only be needed to produce the kinematic coverage and can be drop everywhere else.

The new commodata format allows to have more than 3 kinematic variables, however the parse still parser to load such data.

This would be a very minimal fix that allows to store as many kinematic variables you want, be able to load the data correctly (and eventually access the full kinematics), but keep using in CommonData only the first 3 variables.

Would be this trick be sufficient ? Or am I going to break the entire machine?