Hidden away for decades, many are rarities that had never been seen by curators.The group includes more than 200 works ??? examples by such artists as van Gogh, Degas, Gauguin, Monet and Seurat ??? that were only recently removed from the walls of the Mellons??? many homes, where they were enjoyed by his widow, Rachel Lambert Mellon, who died in March at 103.John Frederick Peto???s ???Still Life w