Rename the dimension `approach` as `model`

[casadoj]

In the COMB (combination of models) experiment, the xarray.Datasets have a dimension called approach with the different combination methods. If I renamed this dimension as model, it would have the same name as in the experiment NWP, which would ease the notebook computing skill.

So far, I had to create an object called model_dim to specify the name of this dimension:

if exp == 'NWP':
    model_dim = 'model'
elif exp == 'COMB':
    model_dim = 'approach'

[casadoj]

I have replaced 'approach' by 'model' this line in 4_confusion_matrix.ipynb:

total_exc = xr.Dataset({
                                '1_deterministic_+_1_probabilistic': deterministic_probabilistic,
                                'model_mean': model_mean,
                                'member_weighted': member_weighted,
                                'brier_weighted': brier_weighted,
                                }).to_array(dim='model')

[casadoj]

Fixed. The only issue is that, now, the confusion matrices in COMB that were computed previously (with the dimension approach) will not work in the notebook that computes skill.