Low interest rates have homeowners swapping out their mortgages for ones with cheaper monthly payments. But while that benefits homeowners, the trend is reducing home-loan profit margins for banks. This week, the nation's largest banks will reveal just how well they are handling razor-thin loan margins. Wells Fargo (WFC) gave analysts a mixed look at what other large institutions, including JPMorgan Chase (JPM) and Citigroup (C) are likely to show