Weight-only systematic variations

Summary of changes:

• Provide a standard interface WeightBase for classes implementing event weights. It supports nominal weights and systematic variations.
• In analysis DileptonTrees, store weights for all registered weight-only systematic variations, in addition to the nominal weight. As a result, most of systematic variations can be computed simultaneously. Dedicated jobs are only needed for variations that change reconstructed objects, such as JEC uncertainty. This behaviour is activated by providing option --syst=weights.
• Steering scripts are updated to support this new option and also use the new behaviour when running DileptonTrees analysis with --syst=all.
• Implement LHE scale variations, independently for the renormalization and factorization scales.
• Include weights for L1T ECAL prefiring.

Validation:

• Checked that nominal weights computed without the L1T prefiring have not changed in a few ZZ→2ℓ2ν events.
• Ran steps 1 and 2 for analysis DileptonTrees.