Add override constraining imports into countries or regions #163

Using this override it's possible to model net self-sufficiency of countries or regions. A new configuration parameter determines the sufficiency levels (e.g. a list of 25%, 75%, 100% by default)

A similar override has been used in this article, see the implementation here: https://github.com/timtroendle/geographic-scale/blob/master/src/construct/import_restrictions.py.