Update plot_risk_maps notebook

[CatalinaYepes]

This PR makes the following changes to the plot_risk_maps scripts and notebooks:

• Add units for the legend
• Add poe in the title for probabilistic maps
• Separate titles for scenario and probabilistic
• Fix bug for log_scale when you have zero losses notebook

This PR addresses some of the points in #23

[raoanirudh]

LGTM!