By Huw Jones LONDON (Reuters) – The London Metal Exchange (LME) took emergency measures to halt trading in nickel on Tuesday as prices doubled to more than $100,000 a tonne. The surge was blamed on short-covering by one of the world’s top producers as Western sanctions squeezed Russia, a major nickel producer. Traders said some holders of nickel positions had also struggled to pay margins to their clearing house. WHAT IS A CLEARING HOUSE? A clearing house is a critical part of the financial market’s basic plumbing, standing between the buyer and seller to take on the risk if one side of the tr…