Add section on proportional symbol maps to the area-data tutorial

[mpjashby]

This should demonstrate that proportional symbol maps might be better than choropleth maps when there are extreme differences in the size of different areas, as illustrated by Andy Wheeler.

[mpjashby]

Adding this to the Week 4 tutorials would make them too long, and it is unlikely this content would fit elsewhere, so there doesn't seem to be a way to fix this.