Choosing the nominal value for the Rochester correction

The best nominal value for the Rochester correction depends on the sample used: the default is ok for MadGraph-based samples, but the alternate is better motivated when using POWHEG samples, see details in slides 13--16 of this talk. The uncertainty is always the difference between the two.

The Rochester correction code currently in the muons branch doesn't allow choosing which entry to use as the nominal value. Support should be added through a new parameter.