By Andy Bruce LONDON (Reuters) - This year now looks unlikely to deliver much improvement in the world economy's growth rate, with a weaker outlook for Europe and the United States tempering the cautious optimism that was evident in January. Still, Reuters polls of roughly 400 economists worldwide, published on Thursday, suggested some of the threats to the health of the world economy are looking muted compared with a year ago. The survey showed the world economy is expected to expand around 3.2 percent this year - the same rate the International Monetary Fund says it grew at in 2012. ...