The Pittsburgh Penguins’ new arena is expected to seat 18,500, or nearly 2,000 more than Mellon Arena now holds for hockey, and will feature a glass atrium that faces downtown Pittsburgh. The atrium will reach as high as 80 feet and will afford those inside a view of downtown. The Penguins on Tuesday showed off the final design made by HOK Sport, which also helped design Heinz Field and PNC Park in…