By Dave Sherwood HAVANA (Reuters) – The number of Cubans leaving their homeland has soared in recent months. The migration, often via irregular and dangerous routes over land and sea, has sparked new tensions between the island´s communist-run government and the United States, the principal destination for Cuban migrants. The long-time adversaries held high-level talks focused on the crisis for the first time in four years in April. WHY ARE SO MANY CUBANS LEAVING THE ISLAND? In November, Nicaragua, a close political ally of Cuba, waived its visa requirement for Cubans, saying the move was inte…