State immunity — Jurisdictional immunity — Nordic Council Agreement on Common Upper Secondary School Education, 1992 — Payment of educational costs by Swedish education authority — Claim to recover costs from Iceland as State of nationality of student — Whether Iceland entitled to jurisdictional immunity — Whether conclusion and implementation of such agreement performed jure imperii or jure gestionis — Whether conclusion of agreement constituting waiver of immunity — Whether contractual choice-of-law clause implying waiver of immunity — The law of Sweden