By Nell Mackenzie, Dhara Ranasinghe and Alun John LONDON, Dec 15 (Reuters) - Trading in the Hungarian forint, long a niche emerging market currency, has more than doubled since U.S. President Donald Trump took office in January, with trader interest only growing since his sweeping "Liberation Day" import tariffs announcement. These increased volumes are no blip either, say traders, strategists and hedge funds navigating the almost $10-trillion-a-day global FX ​markets. The forint has strengthened rough ...