Modified visualize_precision_recall methods to return matplotlib Figure

[FuzzyCoder]

rendering the plots in a dark themed Jupyter notebook Results in unreadable titles (Black on dark). By returning the matplotlib Figure object, the user has the option to modify the plot's appearance (e.g., fig.set_face_color('white').

[MaartenGr]

Thank you for the pull request. There are some that were on purpose and others that might be improved. I'll take some time to go through them and see which ones could be added in the new release.