After 20 years of journeying around the world, travel writer Sylvain Tesson decided he wanted to stay put and fulfil his dream of "living as a hermit." Not one to do things by halves, he spent six months alone in a cabin on the edge of Lake Baikal in Siberia, 75 miles from the nearest village. He took books, cigars and vodka – and discovered space, silence, and solitude