Move downsapling code or use GeoData / DimensionalData #71
Description
The downsampling code we use to simplify the human dispersal problem is too much of a utility to be in this package. We should use aggregate from GeoData instead, or move it to DimensionalData.jl (as I've been meaning to for a while) so we don't have to depend on GeoData.jl here - we already depend on DimensionalData.