“Through May 14, visitors will have a once-in-a-lifetime opportunity to see masterful watercolors by Edwin Abbey, Thomas Eakins, John Singer Sargent, Winslow Homer, John La Farge, Thomas Moran, William Trost Richards, Maxfield Parrish, Jessie Willcox Smith, Violet Oakley, Georgia O’Keeffe, and a host of others.” The Philadelphia Museum of Art exhibition, which opens today, was a bear to put together. Stephan Salisbury tells why.