Example Notebook Plots_of_q2_dependent_observables.ipynb not working

Plotting routines were renamed in flavio, e.g.
`q2_plot_th_diff` --> `diff_plot_th`, which can be resolved easily.

However the new Physics Part throws exception like:
```
AssertionError: Wilson coefficients do not exist in this basis: {'CV_buenu', 'CV_bumunu'}
```
```
ValueError: Only observables that depend on a single variable are allowed
```