VIENNA, May 20 (Reuters) - Iran has been using front companies to skirt international export controls and purchase a graphite compound that can be used in nuclear and conventional arms, an Iranian exile said on Friday. The latest allegation from Alireza Jafarzadeh, an Iranian dissident who has reported accurately on Iran's hidden nuclear facilities and activities in the past, comes days before the European Union meets Iran's top nuclear negotiator in Geneva to persuade Tehran not to resume sensitive atomic work. "Iran has been smuggling into the country a key material that is important to build a nuclear bomb," said...