By Ismail Shakil OTTAWA (Reuters) – Canada, Britain, Sweden and Ukraine on Wednesday called on Tehran to settle a dispute over accountability and reparations for the downing of an airliner by Iranian forces nearly 3 years ago through arbitration under the rules of the 1971 Montreal Convention. All four countries as well as Iran are signatories of the convention, an international treaty which requires states to prevent and punish offences against civil aviation. If the countries cannot settle their dispute within six months, Iran can be taken to the International Court of Justice. Most of the 1…