Add HHModelPinvVHH model.

This PR adds a variant of the HH model with support for the VHH process. The overall rate is now composed of r * [GGF + VBF + VHH] with a corresponding rate parameter r_vhh to control the standalone rate of VHH. The additions to the model are mostly a copy of what is already there for VBF with a few things left to do:

• Verify that the VHH formula is indeed similar to VBF. Right now, it's really just a plain copy.
• Update the cross section values of the VHH samples.
• Check if the label naming scheme is applicable. Maybe it's worth to rename C3 to kl to be fully consistent with the GGF and VBF samples.
• Update tasks and plots for additional r_vhh parameter.

When merged, the new model should perhaps live next to the existing one to avoid interference with the current GGF+VBF combination efforts. In the long term, this combined model might be favorable though.

@lowang @capalmer @lcadamur