By Davide Barbuscia and David Randall NEW YORK (Reuters) – The U.S. Treasury yield curve has been flattening over the last few months as the Federal Reserve prepares to hike rates, and some analysts are forecasting more extreme moves or even inversion. The shape of the yield curve is a key metric investors watch as it impacts other asset prices, feeds through to banks’ returns and even predicts how the economy will fare. Here’s a quick primer explaining what a steep, flat or inverted yield curve means and whether it will forecast the next U.S. recession. WHAT IS THE U.S. TREASURY YIELD CURVE? …