By John McCrank and Davide Barbuscia NEW YORK (Reuters) – U.S. investors holding Russian assets are finding themselves in an increasingly difficult position on working out how to ditch them. The United States, Britain, Europe and Canada announced new sanctions on Saturday – including blocking certain banks’ access to the SWIFT international payment system – following Russia’s invasion of Ukraine. That has sent a wave of investors announcing they are cutting positions in Russia. But investors trying to sell their Russian assets are being left with a problem: How to do it? Russia’s central bank …