Add Doublebtag and JetMass systematics

Added scale factors for double b-tag ParticleNet. The scale factors are included in the event weight (weight_mc) if we are processing a signal sample (with NMSSM in its name) and if both fatjets are b-jets. However, re-weighting will change the overall weight (i.e. not necessarily scaled to the specified lumi anymore). How does one solve this, or do we assume the weighting doesn't have that much of an impact?

Also (accidentally) changed the way the two FatJets are chosen. There are two ways:

1. Check that the jet passes the Ht/eta cuts, THEN check the btag value and see if it has the top 2 largest value
2. Pick the two jets with the largest btag values, THEN check if it passes the cuts

In my tiny QCD test case, when doing Option 1 we get two fatjets 100% of the time, while Option 2 we get two fatjets 85% of the time. Before, we were doing Option 1, but I was restructuring the code (again), and accidentally implemented Option 2. Which one should we do?

Why draft?

1. Have not done extensive testing yet.
2. Have not added JetMass systematics, but can wait for a different PR as well.
3. Don't know if Option 1 or 2 for choosing FatJets is the best.
4. Not sure if I've calculated the scale factors and the up/down correctly :-)
5. Not sure about the weight calculation...