Add JEC uncertainty splits

[cverstege]

Not yet tested, since there's a dcache downtime.

How to use: If an uncertainty file and uncertainty sift is provided, this shift will be used instead of the mean value for JECs.

Values:

• JetEnergyCorrectionUncertaintyParameters (default: empty, path of the uncertainty file)
• JetEnergyCorrectionUncertaintySource (default: "", name of the uncertainty)
• JetEnergyCorrectionUncertaintyShift (default: 0.0, direction and scale of the uncertainty)