Add parameter to customize the report for the number of points into plots

[guillaume-vignal]

By default in the report we have only 200 points into the different plots (in order to have a "light" report in terms of size). In our case to be able to see every modalities in the contribution plot we need to have more points (500 points).

fig = self.explainer.plot.contribution_plot(feature, label=label, max_points=200)