By Olivia Harris ASCOT, England (Reuters) - Specialist police with nuclear and chemical training gave the all clear at the British home of former Russian oligarch Boris Berezovsky on Sunday, a day after the fervent critic of Russian President Vladimir Putin died in unclear circumstances. The 67-year-old, a former powerbroker who helped Putin climb to the top of Russian politics before falling from grace, was found at his house in Ascot, 25 miles west of London. ...