Update `ggsql:world` to use map units instead of countries

[teunbrand]

The current situation in the data is that e.g. the sovereignty of France includes e.g. French Guiana. For plotting purposes it would make sense to separate these so that when one filters FILTER continent == 'Europe', one doesn't include French Guiana in South America.

[teunbrand]

Nope, I've revised my opinion. The map units also splits e.g. the UK into England, Scotland, Wales and Northern Ireland. I'll not comment on the politics of this, but for mapping purposes the UK is compact enough to consider as a single unit, whereas French Guiana is very much spread out from European France.