A Symbol Of Renewal In South London; The Tate Modern, Bright Star On the Thames's Other Side
Tate Modern, London's first authentic modern art museum, is set to open in former power plant; museum provides ample room for old Tate Gallery's 20th-century collection, freeing up old Tate, renamed Tate Britain, for exclusive presentation of British art; museum's role in helping gentrify city's run-down Southwark section helped it raise funds for project; photos (M)