Plot: The energy market has an estimated global value of $6 trillion — a number that could be greatly affected by an increase in renewable sources, especially solar. In his first television project since retiring as host of CBS’s “The Late Show,” David Letterman travels to India for the first time to find out what the world’s soon-to-be most populous country is going to do to expand its inadequate energy grid, power its booming economy and bring basic electricity to 300 million citizens who have never plugged in