Workers from across all four of the Carnegie Museums of Pittsburgh have taken another major step towards unionization by voting to join the Pittsburgh-based United Steelworkers union. Yesterday, it was announced that the United Museum Workers, a group that includes 500 scientists, educators, art handlers, gift shop clerks, and other staff members from Carnegie Museums of Art and Natural History, The Warhol Museum, and the Carnegie Science Center,  officially became part of the USW.…