Add UN M49 region
Following https://discuss.tryton.org/t/country-grouping/5065 a first step is to add the UN M49 regions that are designed for reporting (like for sale reporting). Also we do not really need to depend on restcountries because by design the UN codes are immutable (they create new codes in case of changes). So we could "hard-code" the link between a country and the region. For sale we could add a new level to the report per region to include the UN M49 regions.