Add task for benchmark and c2 scan workflows.

This PR adds the tasks and tools necessary to compute and plot limits for

• EFT benchmarks, and
• a c2 scan.

The biggest difference w.r.t. the usual limit task is that the usual --datacard parameter cannot be used since it would not allow to combine cards of the same benchmark or with the same c2 value across channels. The --multi-datacards parameter should be used instead. The mechanism for combining datacards with same names is explained on the new EFT limits documentation page.

@fmonti @agbet