International organizations — Officials — Immunity — Exemption from taxation — Whether official can be regarded as taxpayer in receiving State — Loan made by international official to property company in receiving State — Whether interest paid to official deductible by company for tax purposes — Difference in treatment between diplomatic missions of foreign States to international organizations and officials of such organizations — Whether constituting inequality of treatment — Vienna Convention on Diplomatic Relations, Article 34 — Application to international officials by analogy — The law of Switzerland