Provide geographical_area_id instead of iso_code.

Geographical area id is not always an iso code. It's iso code for
countries, but otherwise it's geographical area group id. So it should
be up to client app to decide when to call this an iso code.

Since the API will be public one day, it is best to expose Taric structure and not invent our own names. There may be other places where such refactorings are needed.