Adding ttbar systematics + bugfixes

- Split theory systematics into TTJets and VJets
- removed JER from MET systematics (let's wait and do it properly once the new ntuples are here)
- implemented most stuff in 03_*

This is not yet ready to be merged. Outstanding issues:
- store the systematics in separate JSON files (currently storing measurement)
- remove all systematics calculation from plot script
- for comparison with ttbar theory systematics remove this systematic from the central result: central_error = sqrt(central_error**2 - ttbar_systematic**2)