single Higgs scaling + BR scaling in HH model.py

[fabio-mon]

• scale single Higgs as a function of CV, kl, kt
• scale branching ratios as a function of CV, kl, kt --> this can be switched off with the option --PO 'scaleBR=false'
• make double assignment of a scaling a WARNING instead of an ERROR: useful in case there is the same physics process for two different years e.g. qqHH_CV_1_C2V_1_kl_1_2016 and qqHH_CV_1_C2V_1_kl_1_2017

The impact was presented at the HH meeting https://indico.cern.ch/event/922778/contributions/3926882/attachments/2066188/3467540/HH_meeting_29_Jun_2020.pdf