Support custom areas of interest data (i.e. public open space)

[carlhiggs]

Some cities have very good official public open space data that may serve local researcher's analysis and stakeholder reporting purposes better than using OpenStreetMap data (see discussion on #415).

We should add functionality to support inclusion of custom area data like this. It shouldn't be too hard, once time if found to add the functions in and tests to verify support for this.

[carlhiggs]

ties in with #397