(Adds missing word in paragraph 3) * West wants Iran to address nuclear suspicions as part of deal * Iran denies allegations of nuclear ******* agenda * Offers site access, but **t to Parchin facility By Fredrik Dahl VIENNA, **v 20 (Reuters) - Tehran has yet to explain away allegations it conducted atomic bomb research, the head of the U.N. nuclear agency said on Thursday, four days before a deadline for Iran and six world powers to reach a deal on the Iranian nuclear programme. As U.S. ...