combine v8.0.0 for CMSSW_10_2_X on CC7

Updates in this tag compared to v7.0.12:

• New documentation pages, using the mkdocs framework. The documentation source is included in the repository as simple markdown files. Users are welcome to make additions and corrections as pull requests to this repo.
• It is now possible to include additional constraint terms for regularisiation when unfolding using combine. Detailed documentation for this is given here.
• The option -S 0 to remove all systematic uncertainties has been removed. Instead, to freeze all constrained nuisance parameters the option --freezeParameters allConstrainedNuisances should be used, which replaces the previous shortcut of --freezeParameters all.
• The possibility to use some old method names has now been fully removed. When setting the -M option, FitDiagnostics, AsymptoticLimits and Significance must be used instead of, respectively, MaxLikelihoodFit, Asymptotic and ProfileLikelihood.
• Nuisance edit selections for bins, processes or systematic names now require a complete string match. For example, nuisance edit add procA binA [...] will no longer match procAB and binAB. Note that regex selections can still be used to match multiple labels, but again are now required to match the full strings.
• Nuisance parameters can now be frozen using attributes that have been assigned to the corresponding RooRealVars. Syntax is --freezeWithAttributes attr1,attr2,...,attrN.
• For Higgs analyses: added YR4 cross sections, branching ratios and partial width uncertainties in data/lhc-hxswg/sm/, as used in HIG-17-031
• [EXPERIMENTAL] For binned analyses using autoMCStats a faster implementation of the vertical template morphing for shape uncertainties can be enabled at runtime with the option --X-rtd FAST_VERTICAL_MORPH. Any results using this flag should be validated carefully against the default.