Get in, get out, get the next deal. That's the code of the ''flipper." Flipping -- buying and quickly reselling houses -- has helped some investors make a killing in booming markets like Boston's western suburbs. But with concerns rising about a slowing housing market, has the world flipped on flippers? Some analysts and brokers think so. * * * Real estate investing by buyers who don't plan on living in their properties is arguably at historically unprecedented levels, Federal Reserve chairman Alan Greenspan said last month in a speech to the American Bankers Association. This suggests that speculative activity...