Refactor model
This PR adds a refactored version of the physics model, and essentially leaves the current model(s) unchanged to allow for a smooth transition in the future.
As different versions of the same code are used in parallel (default model, vhh model, c2 model), it became somewhat cumbersome / error prone to include necessary updates to all files (such as the recently updated QCD scale + mtop uncertainty coefficients). The refactored version directly includes the vhh production mode which is simply disabled for the default model. The c2-based model now inherits from the default one, drastically reducing the amount of custom changes.