Update ggF cross section calculation
This PR updates the get_ggf_xsec function that is currently only used for plotting purposes (but, by extension, also for results directly inferred from plots such as exclusion ranges from limits).
As before, the cross section is calculated based on the GGF formula of the physics model, plugging in predetermined NLO cross section values of the three (or four) ggF samples. I updated these values to the proper NLO values multiplied by the k-factor. When the scaling to NNLO is enabled (which is the default in the function), the returned values now fully comply with the NNLO values recommended by the LHCHXSWG. Example:
from dhi.models.HHModel import get_ggf_xsec
print(get_ggf_xec(kl=2.))
# -> 0.013803...In addition there is now the option to return up/down varied values based on (kl-dependent) scale and (kl-independent) PDF uncertainties. This feature should be used for plotting uncertainties of the theory prediction in the future.