Confirm and document whether or not zonal_statistics works for other geometries

[emlys]

The docstring and implementation of zonal_statistics suggest it's meant for polygons, but it might work on other geometries too. I accidentally ran it on a point veector and got results, but I don't know if they're correct. We should confirm whether or not zonal_statistics produces valid results on other geometries. If so, update the documentation and tests to support this. If not, assert that the input geometries are indeed polygons.