By Martyn Herman RIO DE JANEIRO (Reuters) - Mark Cavendish rode a wave of British track cycling gold to put himself in contention for the omnium race on Sunday with three disciplines remaining. Frenchman Thomas Boudat topped the standings after the scratch race, 4km individual time trial and elimination race with Italian Elia Viviani in second spot, two points behind. Tour de France stage specialist Cavendish produced a scintillating time trial, going second fastest, 11 seconds quicker than at the London world championships when a poor ride in the discipline left his Rio selection hopes in danger. More...