Transfer data files to a separate repo

[mpjashby]

Since some of the data files are also used by the https://github.com/mpjashby/learn-pop/ package of tutorials, it would be better if the data files could be housed in a separate repo. This would also make it easier for other people to use the data, so it would be useful to provide some documentation for each dataset at the same time.

[mpjashby]

This was done last year, with everything moved to https://github.com/mpjashby/crimemappingdata and all links/references updated.