(N)NLO cross sections for diboson samples

[jabuschh]

During our object review it was pointed out by the GEN expert that we should update the following cross sections [pb] for diboson samples:

• WW (LO → NNLO): 75.91 → 118.7 (from 1408.5243 table 1)
• ZZ (LO → NNLO): 12.13 → 16.91 (from 1405.2219 table 1)
• WZ (LO → NLO): 27.56 → 28.55 (W+) + 18.19 (W-) = 46.74 (from 1105.0020 table 8)

I guess we should update our CrossSectionHelper.py accordingly?

Let me know what you think, I can prepare a PR.

[MatthiesC]

I thought we already had this... apparently not. I'm okay with this.

[jabuschh]

Alright, then I will create a PR 👍🏼