By Joshua McElwee and Joan Faus BARCELONA, June 10 (Reuters) - Pope Leo visited Barcelona's Sagrada Familia on Wednesday and inaugurated the newest of its soaring geometric spires that makes the modernist structure designed by Antoni Gaudi the world's tallest church. At a Mass in the vast, light-infused church, Leo called it an "architectural masterpiece", before walking outside to bless the 172.5-metre (566 feet) Tower of Jesus Christ, crowned with a five-storey ceramic cross visible across the Catalan capital.