implement model combination

Hi, the following MR contains the implementation of the model combination, here are some details to be aware of:

• naming conventions datacards: the processe names for all Higgs processes, so single Higgs backgrounds and di-Higgs signals, need to be renamed with an additional {"_13p0TeV", "_13p6TeV"} depending on the run it belongs to. Examples: ggHH_kl_1_kt_1_13p0TeV_hbbhtt, WH_13p0TeV_htt, ttH_13p0TeV_hbb for run2, or similarly qqHH_CV_1_C2V_1_kl_1_13p6TeV_hbbhtt for run3 datacards. This is necessary to assign processes to the proper morphing formulas (HH) and single H scalings.
• on command line: The argument --hh-model takes the parameters hh_model_bbtt.model_default{,run2,run3}, with the default version containing both run2 and run3. Therefore, the standalone run2 /run3 models can do limit scans in fb (as before), while the default one cannot, due to the different xs for different center of mass energies. Limit scans w.r.t. the signal strength modifier and limits at specific coupling points work for all models.
• The basis used for the morphing of ggf is the same as run2, while for vbf one specific basis from the samples created for run3 was taken, BUT as before, it's advantageous to add all signal samples to the datacards, since everything that's not needed will be dropped automatically by the tasks. Of course, the exact vbf basis still needs to be discussed, but it's trivial to change.

@mrieger