ABUJA, Sept 7( Reuters)-Nigeria's main opposition candidates will appeal a tribunal ruling that affirmed Bola Tinubu's victory in a disputed presidential election in February that they claim was marred by irregularities, their lawyers said. Atiku Abubakar of the People's Democratic Party and Labour Party's Peter Obi, who came second and third respectively, had asked the court to cancel the election, alleging everything from vote fraud to failure by the electoral agency to post results electronic