Add physics model for c2-kl-kt scans, update tasks.

This PR adds a new physics model that can be used to perform fits in the c2-kl-kt space. Since it adds a new parameter, whose SM value is not 1 (unlike all other parameters so far), a few generalizations regarding tasks parameters and plots were required. There are now three different models (default, default + vhh, c2), so further effort is required at some point to reduce the maintenance work and to prevent mistakes (e.g. when updating cross section values, or adding new model features), but this is the target of a future PR.

The now deprecated, dedicated c2 EFT tasks have been removed and the docs are updated.

@tolange