By Michael Weissenstein, Associated Press HAVANA — Fidel Castro thanked Cubans for their well-wishes on his 90th birthday on Saturday and criticized President Barack Obama in a lengthy letter published in state media. “I want to express my deepest gratitude for the show of respect, greetings, and praise that I’ve received in recent days, which gave me strength to reciprocate with ideas that I will send to party militants and relevant organizations,” he wrote. “Modern medical techniques have allowed me to scrutinize the universe,” wrote Castro, who stepped down as Cuba’s president 10 years ago after suffering a severe gastrointestinal illness. Castro accompanied his thanks with reminiscences about his childhood and youth in eastern Cuba, describing the geology and plant life of the region where he grew up. He touched on his father’s death shortly before his own victory in overthrowing U.S-backed strongman Fulgencio Batista in 1959. Castro returns at the [...]