Fix baseline positions on subplots for local explanations (BreakDown, Shap)

[krzyzinskim]

Currently, the default starting x point for the bars on the plots for BreakDown and Shap is the average prediction (it is applied for baseline = None parameter) .

However, the baseline is set only for the first explained prediction and the same one is used for each subsequent explanation on the same plot, which raises problems primarily when considering explainers for different models/classes.

Below is an example of what the explanation for the multiclass model looks like with unbalanced labels. On the left the explanations before the change, on the right - after the change.

[pbiecek]

@krzyzinskim thanks