A Wall Street analyst believes the market is incorrectly discounting Air Products and Chemicals (APD 4.37%), creating a buying opportunity. Shares of the gas distributor are up more than 4% as of 1 p.m. ET on the upgrade.Taking a second look at the businessAir Products is a distributor of essential gases to industrial and medical users. The company in recent years has lived in the shadows of industry leader Linde, and the valuation gap increased recently after Air Products missed on earnings and its stock p ...