Add unit test(s) for demographics export

[kshepard]

Overview

The recent demographics export PR (#162) is perhaps the first case where it would clearly be a good idea to add complementary unit tests. This will be the first test, which means there will need to be some initial effort required to set up a foundation for test data scaffolding (for example, perhaps a small, fake land mass with dummy geolevels and demographic data).

Acceptance criteria

• Unit test(s) are in place that test the key logic of Project demographics export #162