MADRID, July 14 (Reuters) - Spanish renewable power group Opdenergy plans to start trading on July 22 following the approval of its initial public offering by the supervisor, the company said in a statement late on Wednesday. Opdenergy's is planning to set its final price on July 21 after previously determining a price range at between 4.75 euros and 5.40 euros ($4.75 and $5.40) per share, valuing the...