Update the tests. See #44

The slices may specify which region they reference by using its ISO3
code (if it is a country) or its UN_CODE (if it is a region).
Update the parser. See #44

A slice may now use a <country> tag to specify the region it refers to.
Close #44

The link between the slices and the regions is now made in the
helpers layer because it has to query the API to find a region using
its UN_CODE or ISO3.
Merge branch 'enhancement-#44' into develop