Simpler configuration of jet and MET variations

So far all variations were always enabled, but for JER variations it would be useful to be able to e.g. disable them for MET only (see this HN thread ).

With the current changes, the default is "all on" for MC and "all off" for data (removes the unclustered energy variation on MET for data). Removing JER for MC is as simple as passing enableSystematics=lambda v : not v.startswith("jer") (for MC - so more realistically enableSystematics=((lambda v : not v.startswith("jer")) if isMC else None)).

I am using this branch to do some checks that Laurent suggested, so I will keep the PR open to collect more improvements or changes. -> update: seems not to be an urgent priority on the JME side, so merging this (useful config improvement)