Save event weights and systematic variations in Hgg base processor

Added Weight container to event in workflows/base.py, the nominal and systematic variations are now saved to the nominal output file. Changed loop over systematics in base.py to separately treat weight and object systematics.

Added systematics/event_weight_systematics.py and with a dummy example. Added list of available weight systematics to systematics/init.py

Also: scripts/build_corr_syst_jsons.py: First try for a JSON creator based on correctionlib. Only preliminary correction and uncertainties. Use JSON in other scripts to test systematics implementation